ISDOC, PEPPOL and KSeF: How They Fit Together

E-invoicing comes with a lot of acronyms. ISDOC, PEPPOL, KSeF, EN 16931, UBL. They look like competing technologies you have to pick one of. They are not. Each one solves a different layer of the problem, and together they form a single chain: what an invoice is, how it reaches the recipient, and who enforces it.

This text is the untangling. I take each acronym separately, explain what it is for, then show how they fit together. At the end there is a short summary of what to take away if you invoice in the Czech Republic, into Poland, or across the EU.

ISDOC: the Czech national format

ISDOC is the Czech electronic invoice format. It was created under the ICT Union and has been established in the Czech Republic for a long time. Technically it is XML built on the UBL standard, extended with Czech specifics and signed with a XAdES-style XML signature.

In practice ISDOC is dominant for domestic B2B invoicing. Common Czech accounting and ERP systems support it — Fakturoid, Money, Pohoda, ABRA and others. For Czech B2G invoicing (a company invoicing the state), either ISDOC or PEPPOL BIS Billing 3.0 is accepted.

One thing worth stating plainly: ISDOC shares its UBL origin with the European formats, but it is a separate national dialect. It is not automatically conformant with the European standard EN 16931. That means for cross-border use or future EU rules, ISDOC needs to be mapped onto EN 16931, not just sent as-is.

PEPPOL: the European transport layer

PEPPOL is something different from ISDOC. ISDOC is a format — a description of what an invoice looks like. PEPPOL is a network — a way to deliver it securely. These two are easy to confuse, but they solve completely different problems.

PEPPOL is a secure pan-European network for structured B2B and B2G documents. Both sender and recipient connect through certified Access Points, the document is routed through registries (SML and SMP) and transferred over the AS4 protocol. This is called the four-corner model. It is run by the OpenPeppol organisation.

The core idea of PEPPOL is "connect once, reach everyone". Instead of building a connection to each business partner separately, you connect to the network and it delivers to anyone else who is also on the network.

PEPPOL also has its own recommended invoice format — PEPPOL BIS Billing 3.0. This is the most widely used specification for an invoice and credit note. Important: BIS Billing 3.0 is a so-called CIUS of the EN 16931 standard, built on UBL 2.1. Put simply — a valid PEPPOL BIS Billing 3.0 invoice is at the same time conformant with EN 16931. That equality holds, unlike with ISDOC.

PEPPOL is mandatory for B2G in a number of countries (Belgium, Norway, Sweden, Finland, Denmark, the Netherlands and others). On top of that, from 1 January 2026 Belgium mandated PEPPOL for domestic B2B invoicing, with a three-month tolerance until 31 March 2026 without penalties, provided a company shows reasonable effort. PEPPOL is generally treated as the likely transport layer for the whole period the EU is heading toward.

KSeF: the Polish government system

KSeF (Krajowy System e-Faktur) is something different again. It is neither a format nor a network. It is a central government platform — a clearance system. An invoice passes through the government system, which validates it and assigns it a KSeF ID. Only then is it valid. The format is structured XML FA(3).

The key difference from PEPPOL: with PEPPOL the invoice goes directly between companies over the network, and the state does not see it in real time. With KSeF the invoice goes to the state first, the state passes it through, and only then does it exist. This is called the clearance model.

KSeF is mandatory in Poland and the deadlines are confirmed (the law was signed by the president in September 2025):

• 1 February 2026 — mandatory for large taxpayers (2024 gross revenue above 200 million PLN, roughly 4,200 entities). From this date, in addition, everyone must be able to receive invoices via KSeF, even if they do not yet have to issue them themselves.
• 1 April 2026 — mandatory issuance for all other established taxpayers (except the smallest micro-segment).
• 1 January 2027 — the smallest taxpayers come in too (monthly gross revenue up to 10,000 PLN and individual invoices up to 450 PLN — both conditions apply). From this date, penalties for KSeF errors also start running.

Watch out for two things that easily merge into one. The deferral of penalties for KSeF errors until 1 January 2027 (confirmed in the Polish Ministry of Finance Q&A of 3 February 2026) is one thing. The transitional reliefs are another: small companies may use paper and other e-invoices until 30 September 2026, offline mode is allowed until 31 December 2026, invoices from fiscal cash registers are an exception until the end of 2026. These are different dates with different scope, not a single date.

KSeF has a history of postponements — it was originally meant to run in July 2024, but an audit found errors and the infrastructure was rebuilt (KSeF 2.0). The February and April 2026 deadlines are currently confirmed, and according to available sources the February phase for large taxpayers is already running in production. For operational reliability, always verify the status against the Polish Ministry of Finance — but for the February phase the risk of a reversal is now practically gone.

And what about the Czech Republic and the EU as a whole

Above all of this sits ViDA — "VAT in the Digital Age", the EU package that amends the VAT directive. ViDA is the force that will, over time, push the whole EU and their ERP systems toward the EN 16931 standard and near-real-time reporting. I wrote about it in detail elsewhere; here two dates are enough.

• 1 July 2030 — e-invoicing per EN 16931 becomes mandatory for cross-border B2B within the EU. The invoice must be issued within 10 days of the taxable supply, with digital reporting at the moment of issuance. (It is not literal real time — the operational number is those 10 days.)
• 1 January 2035 — existing national systems introduced before 1 January 2024 (the Italian SdI, the Polish KSeF, the French system) must align with the ViDA model and EN 16931.

And now the Czech Republic, because this is where there is the most confusion. The Czech Republic today has no national B2B mandate. B2B e-invoicing is voluntary and requires the recipient's consent. Czech efforts are at an early stage and will likely come only after 2030, driven by ViDA, not as a Polish-style clearance system. For B2G, ISDOC or PEPPOL is accepted. The planned EET 2.0 (sales reporting) is a separate matter — do not confuse it with mandatory B2B invoicing. If someone tells you "mandatory in the Czech Republic from 2027", that is a mix-up with EET 2.0 — a firm date for a Czech B2B mandate does not yet exist.

How it fits together

Putting it all together produces a hierarchy. At the top is the semantic standard, at the bottom the national dialects:

• EN 16931 — the European standard, what an invoice semantically contains.
• PEPPOL BIS Billing 3.0 — a CIUS of this standard on UBL, the default format for cross-border exchange. Conformant with EN 16931.
• ISDOC (the Czech national UBL dialect) and KSeF FA(3) (the Polish national XML) — national formats that over time must be mapped onto EN 16931.

And the roles of the individual layers:

• PEPPOL is the transport layer — how an invoice is delivered, across countries.
• ISDOC is the Czech format — what an invoice contains, in the Czech dialect.
• KSeF is the Polish government system — the clearance gate a Polish invoice must pass through to exist at all.
• ViDA is the EU enforcement force — no near-term Czech mandate, but hard European deadlines (2030, 2035) that push everyone in one direction.

They are not competitors. It is a layered chain: format → transport → enforcement.

What a company should take away

It depends on where you invoice.

You invoice only in the Czech Republic, B2B. Nothing forces you today. ISDOC is established, B2B is voluntary and requires the recipient's consent. The pressure will come from ViDA, likely only after 2030. There is no reason to panic, but it is sensible, when choosing an accounting or ERP system, to verify it can do EN 16931 and PEPPOL — not just ISDOC.

You invoice the state (B2G). You need ISDOC or PEPPOL BIS Billing 3.0. If your system can do PEPPOL, you are covered for the future too, because PEPPOL reaches further than Czech B2G.

You invoice into Poland or have a Polish entity. This is a live, hard deadline. KSeF is mandatory now. Even if you do not have to issue yet, from February 2026 you must be able to receive via KSeF. Large taxpayers issue from February, the rest from April 2026. Enforcement with penalties from January 2027. This is not "sometime in the future" — deal with it now.

You invoice cross-border within the EU. Aim for PEPPOL and EN 16931. That is the default format everything is converging on. Belgium already mandated B2B over PEPPOL from 2026, and from 2030 a cross-border B2B mandate comes across the EU.

A practical rule: do not pick a format, pick a system that can do several formats and convert between them. An invoice you cannot map onto EN 16931 is a debt you pay later.

FAQ

Is ISDOC the same as PEPPOL?

No. ISDOC is an invoice format (what the XML looks like), PEPPOL is a network for delivering it (how it reaches the recipient). You can have an invoice in the PEPPOL BIS Billing 3.0 format and send it over the PEPPOL network. ISDOC, by contrast, is a Czech dialect that is not commonly sent over PEPPOL and is not automatically conformant with the European standard EN 16931.

Does a Czech company have to deal with KSeF?

Only if it invoices into Poland or has a Polish entity. KSeF is a Polish national system, not a Czech one. But note — from 1 February 2026, anyone trading with mandatory Polish taxpayers must be able to receive invoices via KSeF, even if they do not issue invoices via KSeF themselves. For purely domestic Czech invoicing, KSeF does not concern you.

When will e-invoicing be mandatory in the Czech Republic?

A firm date does not yet exist. The Czech Republic today has no national B2B mandate — B2B e-invoicing is voluntary and requires the recipient's consent. The pressure will come from the EU's ViDA package, under the current wording likely only after 2030. The claim "mandatory from 2027" confuses B2B invoicing with the planned EET 2.0 (sales reporting), which is a separate matter.

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We have production experience with KSeF and with conversions between formats. If you are dealing with a connection to the Polish system or cross-border invoicing per EN 16931, get in touch — we will go through what actually concerns you and what is just noise for now.