When a self-billing Invoice or CreditNote message is enveloped, the value of the SBDH element Sender/Identifier SHALL match the AccountingCustomerParty/Party/EndpointID element value, including the value of its schemeID attribute.
/p1:StandardBusinessDocument/p1:StandardBusinessDocumentHeader/p1:Sender/p1:Identifier[$isSelfBilling]. = concat(/p1:StandardBusinessDocument/*/cac:AccountingCustomerParty/cac:Party/cbc:EndpointID/@schemeID, ':', normalize-space(/p1:StandardBusinessDocument/*/cac:AccountingCustomerParty/cac:Party/cbc:EndpointID))