Each line must have an identifier that is unique within the document to make it possible to positively reference the line. For example, from other documents.

Cardinality
1..1
Namespace
cbc urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2
Example value
ND-19245
Rules
Identifier/Error message Flag
BII2-T19-R024
A catalogue line MUST contain a unique line identifier
fatal