Mandatory attribute. Use BT-5 or BT-6.

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