cac:AllowanceCharge[cbc:ChargeIndicator=true()]/cac:TaxCategory[normalize-space(cbc:ID)='AE'][cac:TaxScheme/normalize-space(upper-case(cbc:ID))='VAT'][BR-AE-07]-In a Document level charge (BG-21) where the Document level charge VAT category code (BT-102) is "Reverse charge" the Document level charge VAT rate (BT-103) shall be 0 (zero).
Hitting this rule in production? Paste the invoice into the validator or wire the API into CI so it never ships broken again.