In a Document level allowance (BG-20) where the Document level allowance VAT category code (BT-95) is "Standard rated" the Document level allowance VAT rate (BT-96) shall be greater than zero.
cac:AllowanceCharge[cbc:ChargeIndicator=false()]/cac:TaxCategory[normalize-space(cbc:ID)='S'][cac:TaxScheme/normalize-space(upper-case(cbc:ID))='VAT'](cbc:Percent) > 0