Message

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

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