Glossary

Core terms

Term	Description
AT (Autoridade Tributária e Aduaneira)	Portuguese Tax Authority responsible for tax compliance and regulation.
ATCUD (Código Único de Documento)	Unique invoice identifier combining the AT validation code and a sequential number.
Certified Software	Invoicing software approved by the AT that ensures compliance with Portuguese fiscal rules.
Fiscally Relevant Documents	Documents with fiscal impact, including invoices, receipts, credit notes, and transport documents.
QR Code	Mandatory code on invoices containing structured fiscal data for validation and reporting.
SAF-T (Standard Audit File for Tax)	Standardized XML file used to report accounting and invoicing data to the AT.
Series Validation Code	Code assigned by AT when a series is registered; part of the ATCUD.
Taxable Person	The company or individual legally responsible for issuing invoices.
POS Provider	Provider of the point-of-sale or billing system used to issue invoices and receipts.
Qualified Electronic Signature (QES)	A legally equivalent digital signature required for invoices starting in 2027 (unless using structured e-invoicing like EDI).

Series (Document Series)

A structured sequence of invoice numbers:

• Must be registered with AT before use
• Must be sequential and continuous
• Any change creates a new series