The element "Debited account identifier" (BT-91) should contain a valid IBAN if code 59 SEPA is provided in "Payment means type code" (BT-81).
(/ubl-invoice:Invoice/cac:PaymentMeans[cbc:PaymentMeansCode = 59] | /ubl-creditnote:CreditNote/cac:PaymentMeans[cbc:PaymentMeansCode = 59])[$supplierCountryIsDE and $customerCountryIsDE]not(cbc:PaymentMeansCode = '59') or matches(normalize-space(replace(cac:PaymentMandate/cac:PayerFinancialAccount/cbc:ID, '([ \n\r\t\s])', '')), '^[A-Z]{2}[0-9]{2}[a-zA-Z0-9]{0,30}$') and xs:decimal(string-join(for $cp in string-to-codepoints(concat(substring(normalize-space(replace(cac:PaymentMandate/cac:PayerFinancialAccount/cbc:ID, '([ \n\r\t\s])', '')),5),upper-case(substring(normalize-space(replace(cac:PaymentMandate/cac:PayerFinancialAccount/cbc:ID, '([ \n\r\t\s])', '')),1,2)),substring(normalize-space(replace(cac:PaymentMandate/cac:PayerFinancialAccount/cbc:ID, '([ \n\r\t\s])', '')),3,2))) return (if($cp > 64) then string($cp - 55) else string($cp - 48)),'')) mod 97 = 1