Message

Allowance/charge ChargeIndicator value MUST equal 'true' or 'false'

Context
ubl:Invoice/cac:AllowanceCharge | ubl:Invoice/cac:InvoiceLine/cac:AllowanceCharge | cn:CreditNote/cac:AllowanceCharge | cn:CreditNote/cac:CreditNoteLine/cac:AllowanceCharge
Test
normalize-space(cbc:ChargeIndicator/text()) = 'true' or normalize-space(cbc:ChargeIndicator/text()) = 'false'