For years, many cross-border structures have been modelled around a familiar question: what tax rate applies where the income is received?

A financing company may sit in one jurisdiction, an IP or service company in another, while operating subsidiaries make interest, royalty, guarantee or service payments from multiple countries. If the recipient jurisdiction taxes that income lightly, and the applicable tax treaty restricts withholding at source, the overall structure may look commercially attractive.

The OECD’s Subject to Tax Rule, or STTR, changes part of that equation.

On 15 September 2026, Albania deposited the second instrument of ratification for the STTR Multilateral Convention, following San Marino. That second ratification means the Convention itself will enter into force on 1 January 2027. The STTR allows a source jurisdiction, in defined circumstances, to “tax back” certain intra-group payments where the income is subject to a corporate tax rate below 9% in the recipient jurisdiction. (OECD)

But the important message for businesses is not that every low-tax structure suddenly changes on 1 January 2027.

It is this:

A low tax rate at the recipient level can no longer be analysed separately from the treaty rights of the country making the payment.

The STTR Is Not Another Version of the 15% Global Minimum Tax

This distinction is essential.

Pillar Two contains two different concepts.

The better-known GloBE rules broadly seek to ensure that large multinational groups are subject to a minimum effective tax rate of 15% on a jurisdictional basis.

The STTR is different.

It is a tax-treaty rule, focused on specified cross-border payments between connected persons. Instead of testing a group's jurisdictional effective tax rate, it examines the nominal tax rate applicable to a particular category of income in the recipient jurisdiction. If that rate is below 9%, the source country may receive an additional taxing right. (OECD)

This also means the familiar EUR 750 million GloBE revenue threshold should not be used as a shortcut for determining STTR exposure. The STTR has its own connected-party, payment-category and materiality tests.

A multinational outside GloBE can therefore still have an STTR question.

How the 9% Mechanism Works

Assume a group company in Country A pays interest to a connected financing company in Country B.

Country B taxes that interest at 0%.

The treaty currently allows Country A to impose withholding tax of only 5%.

If the treaty incorporates the STTR, the 9% minimum creates a potential additional source-country taxing right.

The STTR calculation effectively starts with:

9% minimum rate – tax rate in recipient jurisdiction

In this example, that produces 9%.

But because the existing treaty already permits Country A to tax the payment at 5%, the STTR does not simply add another 9%. The existing source taxing right is taken into account, leaving a potential additional STTR right of up to 4%, subject to the source country's domestic law and the detailed conditions. (OECD)

Now change the treaty.

If Country A already has the right to tax the payment at 10%, the STTR may add nothing because the treaty already gives the source jurisdiction a taxing right at least equal to the relevant STTR differential. (OECD)

That is why headline statements such as “payments to a 0% jurisdiction will now suffer 9% withholding tax” are too simplistic.

The existing treaty has to be read first.

Which Payments Can Be Caught?

The STTR is broader than interest and royalties.

The OECD's covered categories include:

interest; royalties; payments for distribution rights; insurance and reinsurance premiums; financial guarantee and other financing fees; payments for the use of industrial, commercial or scientific equipment; and income from the provision of services. (OECD)

That breadth makes the rule relevant to many common multinational structures.

A regional treasury company earning interest may be affected.

A holding or finance entity receiving guarantee fees may need review.

A central service company charging management, procurement, technical or administrative fees may also fall within the potential scope.

An IP structure can raise royalty questions.

So this is not merely a financing rule.

Why UAE Structures Deserve Particular Attention

Consider an Indian or global group using a UAE entity as a regional hub.

Under the UAE corporate tax regime, a Qualifying Free Zone Person can currently benefit from 0% corporate tax on Qualifying Income, while non-qualifying taxable income is generally taxed at 9%. (وزارة المالية - الإمارات العربية المتحدة)

That does not mean every payment to a UAE Free Zone company becomes subject to the STTR.

Far from it.

First, the relevant bilateral treaty must actually contain the STTR, whether through the STTR Multilateral Convention or another treaty amendment.

Second, the payment must constitute covered income.

Third, the payer and recipient must meet the connected-person conditions.

Fourth, the tax rate applicable to that specific income must fall below 9% after applying the STTR methodology.

And the exclusions, mark-up threshold and materiality rules must also be considered.

But the broader structuring lesson is important:

A 0% recipient-side tax outcome is no longer enough to conclude that the overall treaty outcome remains attractive.

For groups using UAE, Singapore or other regional hubs, the correct exercise increasingly becomes payment-by-payment and treaty-by-treaty.

The Rule Does Not Apply to Every Intra-Group Service Fee

There are several important filters.

The STTR applies to connected persons. The OECD test generally looks to control or more than 50% direct or indirect beneficial ownership, vote and value, together with broader factual control principles. (OECD)

There is also an important 8.5% mark-up threshold for several categories of covered income, including services, distribution rights, insurance, guarantee fees and equipment rentals.

Where the relevant income does not exceed the associated direct and indirect costs plus an 8.5% mark-up, the STTR generally does not apply to those categories, subject to the detailed anti-avoidance provisions. (OECD)

This is particularly relevant to routine service arrangements.

Suppose a regional service company earns a 5% cost-plus return from a connected operating company and the relevant payment otherwise falls within the covered service-income category.

The 8.5% rule may become important.

But the STTR mark-up test should not be confused with transfer pricing benchmarking. OECD guidance expressly notes that the STTR cost-and-mark-up test is not itself the arm's-length pricing method, although transfer pricing analysis may assist in determining relevant costs. (OECD)

That distinction will matter in practice.

A 5% transfer pricing mark-up and an 8.5% STTR threshold answer different questions.

There Is Also a Materiality Threshold

The STTR is not intended to capture every small payment.

For contracting states with GDP of at least EUR 40 billion, the annual materiality threshold is generally EUR 1 million. Where the relevant GDP test is below EUR 40 billion, the threshold is EUR 250,000. Covered payments between connected persons are aggregated for this purpose in accordance with the rule. (OECD)

For multinational treasury and service structures, this means the review should be performed at an aggregated treaty-flow level rather than by looking at one invoice in isolation.

The Tax Is Not Designed as Immediate STTR Withholding

Another practical point is often overlooked.

The OECD framework provides for the STTR charge to be determined after the end of the relevant fiscal year, because several conditions, including materiality and mark-up tests, cannot necessarily be established when an individual payment is made.

The STTR itself therefore operates through an ex-post annualised charge, rather than requiring the additional STTR amount to be collected immediately when each payment is made. Ordinary withholding that already applies under domestic law or other treaty provisions is a separate matter. (OECD)

For finance teams, this creates a new year-end tax provisioning and data requirement.

1 January 2027 Does Not Mean Every Treaty Changes on That Date

This point deserves particular emphasis.

Albania's ratification means the STTR Convention itself enters into force on 1 January 2027, initially for Albania and San Marino as ratifying parties. (OECD)

That is not the same as saying every treaty containing one of those jurisdictions immediately starts charging STTR tax from that date.

The Convention contains separate entry-into-effect rules. Broadly, the STTR provisions apply to a covered treaty only after the Convention is in force for both relevant contracting jurisdictions and, generally, for fiscal years beginning after a six-calendar-month period following the later of those entry-into-force dates. (OECD Legal Instruments)

So the practical application will roll out progressively as additional jurisdictions sign, ratify and match treaties.

For Indian groups, the right response today is therefore mapping, not assuming.

India and the Broader Source-Country Trend

The STTR also fits into a wider international-tax debate.

The OECD rule gives developing source countries an additional treaty right where specified related-party income is lightly taxed at destination.

The UN Model has long taken a comparatively source-oriented approach. Its Article 12A, for example, allows source taxation of qualifying fees for technical services without requiring a permanent establishment, while Article 12B deals with automated digital services. (Financing for Development)

These are not the same rules as the STTR.

But they reflect a similar policy reality: source countries increasingly want to retain greater taxing rights over payments arising from their economies.

That direction matters for Indian multinationals both inbound and outbound.

What Should Groups Do Now?

The first exercise should be relatively simple.

Map all material cross-border payments between connected entities:

interest;

royalties;

service fees;

guarantee and financing fees;

equipment rentals;

distribution-related payments; and

other covered flows.

Then overlay four questions.

What treaty applies?

Has or could that treaty incorporate the STTR?

What nominal tax rate applies to that specific income in the recipient jurisdiction?

What source tax does the treaty already permit?

After that, apply the connected-party, mark-up, materiality and exclusion tests.

This should ideally be done together with transfer pricing, withholding-tax and legal-entity reviews rather than as a separate Pillar Two exercise.

Closing Perspective: Re-Test the Cash Flow, Not Just the Entity

Low-tax regional hubs will continue to have commercial roles.

Treasury, procurement, IP, regional management and service functions do not suddenly become inappropriate because the STTR exists.

But the economics may change.

Historically, international structuring often began by asking:

“Where should the entity sit?”

The next phase will increasingly require another question:

“What payments will flow through it, under which treaties, and how are those specific payments taxed?”

That is the practical significance of the STTR.

The structure may not have changed. The treaty economics might.

For groups using low-tax regional entities, 2027 should therefore not be viewed simply as the year a new OECD convention enters into force.

It should be treated as the point to begin re-testing whether the historical assumptions behind cross-border interest, royalty, service and guarantee flows still hold.

Research basis

This article is based principally on the OECD's September 2026 announcement on the STTR Convention, the OECD STTR model provision and commentary, the STTR Multilateral Convention and explanatory statement, current UAE Ministry of Finance/Federal Tax Authority materials, and the UN Model Tax Convention framework. The STTR is treaty-specific and its practical application depends on the relevant jurisdictions, treaty coverage, ratification status, domestic law and individual payment facts as of 21 September 2026.