Mandatory attribute. Use BT-5

Example value
SGD
Code lists
ISO 4217 Currency codes
Rules
Identifier/Error message Flag
PEPPOL-EN16931-CL007
[PEPPOL-EN16931-CL007]-Currency code must be according to ISO 4217:2005
fatal
PEPPOL-EN16931-R051
[PEPPOL-EN16931-R051]-All currencyID attributes must have the same value as the invoice currency code (BT-5), except for the invoice total GST amount in accounting currency (BT-111)
fatal