Must be according to "PEPPOL policy for identifiers", example GLN.

Usage
Mandatory
Rules
Identifier/Error message Flag
EUGEN-T01-R012
A party identifier MUST have a scheme identifier attribute
fatal
OP-T01-R005
An Party Identifier Scheme identifier MUST be from the list of PEPPOL Party Identifiers described in the "PEPPOL Policy for using Identifiers".
fatal