The headline number in Pillar Two is easy to remember: 15%.

The difficult part is everything required to prove whether a multinational group has actually reached that minimum rate in every relevant jurisdiction.

For many CFOs, tax heads and finance teams, the real Pillar Two challenge is therefore becoming less about understanding the Global Anti-Base Erosion—or GloBE—rules and more about answering a practical question:

Can the organisation produce the right data, for the right legal entities, using consistent definitions, within the required filing timeline?

That question became even more important on 11 September 2026, when the OECD/G20 Inclusive Framework released its latest Global Minimum Tax implementation package. It includes an updated GloBE Information Return (GIR), a framework for full legislative peer reviews and further administrative guidance. The revised GIR incorporates the simplifications agreed under the January 2026 Side-by-Side package and applies to fiscal years beginning on or after 31 December 2025. The OECD is now developing a revised XML schema to reflect those changes. (OECD)

The message for multinational groups is clear: Pillar Two has moved well beyond policy design. It is now an operational reporting, systems and data-governance exercise.

First, What Does Pillar Two Actually Do?

The GloBE rules broadly apply to multinational groups with consolidated revenue of at least EUR 750 million in at least two of the four fiscal years immediately preceding the tested year. The threshold is based on the consolidated financial statements of the ultimate parent entity. (OECD)

For an in-scope group, the rules test the effective tax rate on a jurisdiction-by-jurisdiction basis.

Broadly, the group aggregates GloBE income or loss and adjusted covered taxes of constituent entities in the same jurisdiction to determine that jurisdiction's effective tax rate. Where it is below 15%, top-up tax may arise after applying the relevant GloBE adjustments and the substance-based income exclusion. That exclusion itself uses eligible payroll costs and tangible assets. (OECD)

The collection mechanism also matters. A Qualified Domestic Minimum Top-up Tax, or QDMTT, generally gives the low-tax jurisdiction the first opportunity to collect the top-up. The Income Inclusion Rule, or IIR, can then operate higher in the ownership chain, with the Undertaxed Profits Rule, or UTPR, acting as a backstop where the relevant top-up is not otherwise collected. (OECD)

On paper, that sounds like a tax calculation.

In practice, it is a data architecture problem.

Why the 15% Calculation Is More Complicated Than the Statutory Tax Rate

Consider a multinational group with entities in India, Singapore, Germany, the UAE and the United States.

Management might know each country's headline corporate tax rate. But Pillar Two does not simply ask whether that statutory rate exceeds 15%.

It requires a separate GloBE computation involving financial accounting income, prescribed adjustments, covered taxes, deferred tax positions, ownership structures and—in appropriate cases—payroll and tangible assets.

The GIR reporting architecture includes information concerning constituent entities and ownership interests, jurisdictional GloBE calculations, adjusted covered taxes and several other calculation elements. The reporting framework also requires detailed information relevant to the substance-based income exclusion, including eligible payroll and tangible assets. (OECD)

That information rarely sits in one system.

The consolidation team may own the accounting numbers. Local tax teams may own current tax data. Deferred taxes may sit in a tax-provision tool. Employee information comes from payroll or HR systems. Fixed-asset registers may operate locally. Legal owns the entity chart. M&A teams may hold acquisition-date information.

Pillar Two asks those datasets to speak the same language.

The Hidden Problem: One Number Can Have Several Versions

Take something apparently simple—tax expense.

The amount shown in a local statutory account may differ from the amount recorded in the group reporting package. Current tax may differ from cash tax paid. Deferred tax balances may require GloBE-specific treatment. Post-filing adjustments can change covered taxes relating to earlier periods.

OECD guidance expressly recognises that subsequent tax adjustments can change the effective-tax-rate and top-up-tax outcome for an earlier year. (OECD)

Now multiply that issue across 50 jurisdictions and perhaps several hundred legal entities.

The tax calculation itself may be complex, but the more fundamental question becomes:

Which number is authoritative, where did it come from, and can the group reproduce it next year?

A spreadsheet assembled manually from emails may produce a technically correct answer once.

That is not the same as having a sustainable Pillar Two compliance process.

Acquisitions Make the Data Problem Even Harder

Imagine the group acquires a company midway through the year.

Pillar Two now needs to understand when that entity joined the group, how ownership changed and what portion of the relevant financial information belongs within the reporting period.

The GIR framework contains specific ownership reporting, including changes in ownership interests. OECD guidance also provides rules for adjusting eligible payroll costs and tangible assets when an entity becomes or ceases to be a constituent entity during the fiscal year. (OECD)

This creates a practical M&A issue.

A transaction may close successfully from the legal and financial perspective, but if the acquired company uses a different ERP, chart of accounts, payroll system and tax-provision process, the Pillar Two integration problem begins immediately.

Tax data therefore needs to become part of post-acquisition integration, not something discovered at year-end.

The GIR Is Turning Pillar Two Into a Reporting Discipline

The GloBE Information Return is the standardised return through which implementing jurisdictions can obtain the information required to assess an MNE group's Global Minimum Tax position.

The September 2026 version incorporates the January Side-by-Side simplifications and will apply only to GIRs for fiscal years commencing on or after 31 December 2025. The OECD has also confirmed that a revised XML schema is being developed, meaning both tax authorities and multinational groups will need to update reporting and filing systems accordingly. (OECD)

This version control matters.

A multinational cannot simply build one static Pillar Two template and assume it will remain unchanged.

The underlying rules, safe harbours, reporting format and domestic implementation requirements continue to evolve.

Safe Harbours Reduce Calculations—Not the Need for Good Data

The January 2026 Side-by-Side package introduced important simplifications.

These include a Simplified ETR Safe Harbour, a one-year extension of the Transitional CbCR Safe Harbour, a Substance-Based Tax Incentive Safe Harbour and additional safe harbours for qualifying groups headquartered in jurisdictions meeting specified minimum-tax requirements. The package does not displace the primacy of QDMTTs. (OECD)

That can materially reduce compliance burdens in qualifying circumstances.

But a safe harbour is not the same as “no data required”.

The group still has to determine whether it qualifies for the simplification. That requires reliable information and consistent classification.

In fact, simplification can create another governance requirement: the organisation needs to know which jurisdictions use the full GloBE calculation, which qualify for a safe harbour, which elections were made and whether those conclusions remain valid in the following year.

A decentralised multinational can therefore face different Pillar Two processes across different countries within the same reporting period.

Filing Deadlines Leave Less Time Than Many Finance Teams Expect

Under the OECD Model Rules, the GIR is generally due 15 months after the end of the reporting fiscal year, extended to 18 months for the transition year. For a calendar-year multinational first entering the rules for the year ending 31 December 2024, the model timeline therefore resulted in a first GIR deadline of 30 June 2026. Domestic rules ultimately determine the actual legal filing obligation in each implementing jurisdiction. (OECD)

Those first filing cycles have therefore already started.

The OECD has also acknowledged implementation challenges around filing portals and exchange arrangements. In May 2026, jurisdictions implementing the Global Minimum Tax from 2024 agreed mechanisms intended to preserve the benefits of central GIR filing where local portals or exchange arrangements were not yet fully operational. (OECD)

This reinforces an important distinction.

Pillar Two readiness is not something groups can solve comfortably after the financial statements are signed. By then, several required data decisions may already be embedded in systems and closing processes.

What Should CFOs Be Asking Now?

The starting point should be a Pillar Two data map, not another technical memorandum.

A practical review should identify:

• every constituent entity, permanent establishment and relevant ownership relationship;

• the source system for financial accounting income, current and deferred taxes, payroll and tangible assets;

• differences between local accounts, consolidation packages and tax-return data;

• jurisdictions relying on safe harbours versus full calculations;

• responsibility for elections, adjustments and documentation;

• acquisitions, disposals and restructurings that change entity or ownership data; and

• how the final GIR numbers can be traced back to source records and reproduced during an audit.

The objective is not merely automation.

It is auditability.

A sophisticated Pillar Two engine fed by unreliable source data will calculate unreliable results faster.

The September 2026 OECD Package Raises the Governance Bar

The other important element of the 11 September package is the new framework for full legislative reviews.

Inclusive Framework members will undertake detailed peer reviews of jurisdictions' IIR, UTPR and QDMTT legislation to assess consistency with the GloBE Model Rules and Commentary. Where inconsistencies arise, recommendations may be issued. (OECD)

For multinational groups, this means domestic implementation cannot simply be viewed as a collection of unrelated local taxes.

The architecture is deliberately becoming more coordinated.

That improves certainty over time, but it also increases the importance of maintaining a single defensible global data position.

If one jurisdiction receives one version of the group's Pillar Two story while another receives a materially different version, questions become inevitable.

Closing Perspective: Pillar Two Is Becoming a Finance Transformation Project

The global minimum tax is usually introduced as a 15% tax rule.

That description increasingly understates the challenge.

For large multinational groups, Pillar Two requires finance, tax, legal, HR, treasury, consolidation, M&A and technology teams to produce a consistent view of the same global business.

The calculation may identify whether a jurisdiction owes top-up tax.

But the quality of the underlying data determines whether that calculation can be defended.

The September 2026 GIR update is therefore a useful reminder that the organisations best prepared for Pillar Two will not necessarily be those with the most elaborate tax models.

They will be the groups that can answer three simple questions confidently:

Where did this number come from? Who owns it? And can we reproduce it tomorrow?

That is why, for many multinational CFOs, the hardest part of the 15% Global Minimum Tax may ultimately have very little to do with the number 15%.

Research cut-off: 14 September 2026. The regulatory positions above are based principally on OECD/G20 Inclusive Framework materials, including the September 2026 GIR and implementation package, the 2026 Consolidated Commentary and the January 2026 Side-by-Side package.