Message

A VAT number MUST be valid Norwegian organization number (nine numbers) followed by the letters MVA.

Context
cbc:CompanyID[@schemeID = 'NO:VAT'] | cac:PartyTaxScheme/cbc:CompanyID[not(@schemeID)]
Test
matches(., '^[0-9]{9}MVA$') and u:mod11(substring(., 1, 9))