For Swedish suppliers, Swedish VAT-numbers must consist of 14 characters.
//cac:AccountingSupplierParty/cac:Party[cac:PostalAddress/cac:Country/cbc:IdentificationCode = 'SE' and cac:PartyTaxScheme[cac:TaxScheme/cbc:ID = 'VAT']/substring(cbc:CompanyID, 1, 2) = 'SE']
string-length(normalize-space(cac:PartyTaxScheme[cac:TaxScheme/cbc:ID = 'VAT']/cbc:CompanyID)) = 14