> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntage.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Invoice relations

Invoice relations represent CFDI relationships between invoices, such as substitutions, credit notes, or other SAT-defined relation types. Each relation includes the source invoice, related invoice when Syntage has it, related invoice UUID, and the SAT relation type code.

Use invoice relations when an integration needs to explain why one invoice references another invoice. You can retrieve a relation by ID, list relations for an invoice, or list relations for an invoice within an entity.

Invoice relations are extracted from invoice data when the CFDI includes related-document metadata. To retrieve them, run an [`invoice` extraction](/resources/datasources/sat/invoices#extracting-invoice-data) for the entity, then list relations after the extraction finishes.