UBL-SR-59
A Financial Institution Branch shall contain exactly one Payment account identifier (BT-84).
What the rule checks
As the rule set's publisher wrote it. The rule runs on every element the context matches, and fails where the test is false; the validator reports the line of that element.
cac:PaymentMeans
not(cac:PayeeFinancialAccount) or count(cac:PayeeFinancialAccount/cbc:ID) = 1
Checked with EN 16931 Invoice (UBL), EN 16931 Credit Note (UBL), Croatia eRačun Invoice (UBL), Croatia eRačun Credit Note (UBL)