Invoice Series: What They Are and How to File Them With the Tax Authority
What an invoice series is, why document types must be kept apart, and how to file a series with the Portuguese tax authority to obtain an ATCUD.
If you issue invoices in Portugal, whether through certified software or straight from a Shopify store or Stripe payments, you will inevitably run into the concept of an invoice series. It looks like a technical detail, but the series is what guarantees the legal integrity of all your invoicing and what lets the tax authority identify every document you issue.
This article explains what a series is, why you have to keep them apart, and how to file them to obtain the ATCUD validation code.
What an invoice series is
A series is a continuous numeric sequence, identified by a name, holding documents of the same type issued by the same certified software. Each document inside the series receives a unique, immutable sequence number.
The combination of document type + series name + sequence number uniquely identifies any tax document in Portugal. For example:
FT 2026A/00042
Read: invoice (FT), from series 2026A, number 42 within it.
That identifier appears:
- On the printed document, at the top or beside the QR Code.
- Inside the mandatory QR Code.
- In the ATCUD (
series-code-number). - In the SAF-T (PT) file filed monthly.
Why you have to keep series apart
Portuguese law does not allow mixing document types in one series. Each type (FT, FR, FS, NC, ND, GT and so on) needs its own numeric sequence. Beyond that, it is good practice to separate by:
- Financial year, so numbering restarts in January and year-end closing is cleaner.
- Sales channel: one series for Shopify, another for Stripe, another for in-person sales, for instance.
- Establishment, if you have several physical shops.
The golden rule: inside a series there are no gaps and no repeats. If you issue number 42, the next one is 43. You cannot jump to 50, nor reuse 42 later.
What a series name looks like
The name is free, but it has to respect:
- Capitals, digits and a few separators only, generally no spaces and no special characters.
- A reasonable length: most packages accept up to 35 characters, but keep it short.
- An exact match between the name in the software and the name filed with the tax authority. One letter of difference and the SAF-T file is rejected.
Common conventions:
| Pattern | Example | Use |
|---|---|---|
| <YEAR><TYPE> | 2026FT, 2026FR | Most used in e-commerce |
| <TYPE><YEAR> | FT2026, FR2026 | Equivalent, personal taste |
| <CHANNEL><YEAR> | SHOP26, STRIPE26 | When there are several channels |
| Plain <TYPE> | FT, FR | A continuous multi-year series |
ATCUD: the missing piece
Since 1 January 2023, under Portaria 195/2020, every certified tax document has to carry the ATCUD, the unique document code. The ATCUD is made of:
ATCUD: <AT-validation-code>-<sequence-number>
For example: CSDF7T5H-00042.
The validation code (CSDF7T5H) is assigned by the tax authority to the series, not to the document, and is 8 alphanumeric characters. You obtain it by filing the series before issuing the first document. If you have not yet created the AT sub-user for filing, start with ATCUD: creating the user at the tax authority.
How to file a series
The process is centralised on the Portal das Finanças:
- Sign in with the company VAT number, or with the dedicated sub-user, which is recommended.
- Todos os Serviços → Faturas e Outros Documentos Fiscais → Comunicar Séries Documentais.
- Click Registar Série.
- Fill in:
- Document type (FT, FS, FR, NC, ND, GT, GR...).
- Series identifier, for example
2026FR. It has to match the software. - Starting sequence number, normally
1. - Expected start date.
- Processing method:
Programa informático de faturação. - Software certification number: each package has its own.
- Submit. The tax authority returns the 8-character validation code.
- Go back to the software and paste the code into the series' ATCUD field.
Filing can be done by hand on the portal or by webservice, with the software filing the series itself. InvoiceXpress, Moloni and Vendus all support automatic filing: you just authorise the corresponding AT sub-user.
Document types and when to use each
In Portuguese e-commerce, the most frequent are:
| Code | Name | When to issue | |---|---|---| | FT | Invoice | A sale with the customer's VAT number (B2B, or B2C on request) | | FS | Simplified invoice | A B2C sale with no VAT number, up to €1,000 | | FR | Invoice-receipt | A sale already paid at the time, typical of Shopify and Stripe payments | | NC | Credit note | Cancellation, or partial/full refund | | ND | Debit note | An addition to an earlier invoice | | GT | Delivery note | Goods in transit |
Each of these needs its own series. You cannot issue a credit note inside the invoice series.
Yearly series versus continuous series
The tax authority neither requires nor forbids either approach, but accounting practice is divided:
- Yearly series (
2025FR,2026FR, ...): numbering restarts at 1 every year. It means filing a new series each January. Cleaner for audits and for the annual SAF-T close. - Continuous series (
FR, no year): filed once, never touched again. The numbers grow indefinitely. Simpler, but less legible as time passes.
The most common choice in e-commerce is the yearly series, because volume is high and there is value in reading the financial year off the name itself.
Common mistakes
- A name that does not match between the software and the tax authority. It causes immediate SAF-T rejection. Confirm by copy and paste.
- The wrong starting number when migrating from another system. If you already issued 50 invoices in 2026 elsewhere, the new series has to start at 51, or you need a separate migration series.
- Forgetting to file before issuing. An invoice issued in an unfiled series goes out without a valid ATCUD. It is not certified.
- Mixing document types. You cannot have FT and FR in the same series; they are separate filings.
Software-specific guides
Every certified package has its own flow for creating series internally. Follow the guide for yours:
- How to create an invoice series in InvoiceXpress
- How to create an invoice series in Moloni
- How to create an invoice series in Vendus
How Rioko helps
When you connect your Shopify store or Stripe account to Rioko, the dashboard reads every active series from your InvoiceXpress, Moloni or Vendus account through the API. You pick the series to use by default, and Rioko checks the ATCUD is configured before issuing any document, blocking issuing when the series is not certified. At the turn of the financial year you get a warning to configure the new yearly series, so January's invoices do not come out still numbered in last year's.
Original source: invoicexpress.com
Frequently asked questions
Do I really need one series per document type?
Yes. The law does not allow mixing types in the same numeric sequence. Invoices, invoice-receipts, credit notes and delivery notes each need their own series, each filed with the tax authority.
Yearly series or continuous series?
Both are legal. A yearly series restarts the numbering in January and has to be filed again every year; a continuous one is filed once and grows indefinitely. In e-commerce the yearly series is the more common choice, because of volume.
Can I rename a series after issuing?
No. After the first document the series is locked to the name it was filed under. If you got it wrong, create a new series with the right name and deactivate the old one.
I issued invoices in a series that was never filed. What now?
Those documents went out without a valid ATCUD. File the series, then void and reissue the affected documents with credit notes in the now certified series. There is no way to add an ATCUD after the fact.
Can the numbering have gaps?
No. Inside a series there can be neither gaps nor repeats. If number 42 was issued, the next one is 43.
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