# BT-24 — Specification identifier

> https://billhorse.com/en/terms/bt-24/

URN of the specification the invoice was built against — validators derive the profile (XRechnung, Factur-X, Peppol BIS) from it. Without it, profile-specific validation is impossible.

## Where does it live in the XML?

- UBL: `cbc:CustomizationID`
- CII: `rsm:ExchangedDocumentContext/ram:GuidelineSpecifiedDocumentContextParameter/ram:ID`

## Checked by these rules

- [BR-01 — Specification identifier missing](https://billhorse.com/en/rules/br-01/)