Each Document level charge (BG-21) shall have a Document level charge VAT category code (BT-102).
/ubl:Invoice/cac:AllowanceCharge[cbc:ChargeIndicator = true()] | /cn:CreditNote/cac:AllowanceCharge[cbc:ChargeIndicator = true()]
exists(cac:TaxCategory[cac:TaxScheme/normalize-space(upper-case(cbc:ID))='VAT']/cbc:ID)