Validation issue: UBL-SR-53 not enforced for AccountingCustomerParty PartyTaxScheme (missing CompanyID)

[wiknabe]

Description

I believe Mustang Validator has an issue with the correct enforcement of the UBL-SR-53 rule.

According to the Peppol / EN16931 rule set, CompanyID (VAT Identifier) must be stated when providing PartyTaxScheme/TaxScheme/ID.

However, Mustang currently validates an invoice as correct, even though it violates this rule.
When cross-checking the same invoice with the official KoSIT XRechnung Validator, it is correctly rejected.

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Steps to Reproduce

1. Validate the attached XRechnung invoice using Mustang CLI. ubl_incorrect_test.xml
2. Mustang reports the invoice as valid.
3. Validate the exact same file using the KoSIT Validator.
4. KoSIT rejects the invoice due to rule UBL-SR-53.

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Problematic XML Fragment

The invoice contains the following block under cac:AccountingCustomerParty:

<cac:PartyTaxScheme>
    <cac:TaxScheme>
        <cbc:ID>VAT</cbc:ID>
    </cac:TaxScheme>
</cac:PartyTaxScheme>

Mustang Output

java -Xmx1G -Dfile.encoding=UTF-8 -jar Mustang-CLI-2.21.0.jar --action validate --source ubl_incorrect_test.xml [main] INFO com.helger.schematron.xslt.SchematronResourceXSLTCache - Compiling XSLT instance [cpPath=/xslt/en16931schematron/EN16931-UBL-validation.xslt; urlResolved=true; URL=jar:file:/xrechnung/Mustang-CLI-2.21.0.jar!/xslt/en16931schematron/EN16931-UBL-validation.xslt]
[main] INFO com.helger.schematron.api.xslt.AbstractSchematronXSLTBasedResource - Applying Schematron XSLT on XML instance
[main] INFO com.helger.schematron.xslt.SchematronResourceXSLTCache - Compiling XSLT instance [cpPath=/xslt/XR_30/XRechnung-UBL-validation.xslt; urlResolved=true; URL=jar:file:/xrechnung/Mustang-CLI-2.21.0.jar!/xslt/XR_30/XRechnung-UBL-validation.xslt]
[main] WARN com.helger.xml.transform.LoggingTransformErrorListener - [warn] Transformation warning (net.sf.saxon.trans.XPathException: A function that computes atomic values should use xsl:sequence rather than xsl:value-of)
[main] WARN com.helger.xml.transform.LoggingTransformErrorListener - [warn] Transformation warning (net.sf.saxon.trans.XPathException: A function that computes atomic values should use xsl:sequence rather than xsl:value-of)
[main] INFO com.helger.schematron.api.xslt.AbstractSchematronXSLTBasedResource - Applying Schematron XSLT on XML instance
[main] INFO org.mustangproject.validator.ZUGFeRDValidator - Parsed PDF:absent XML:valid Signature:null Checksum:271FB8540559EBEA23E93CD9C2DA9C0505592247 Profile:urn:cen.eu:en16931:2017#compliant#urn:xeinkauf.de:kosit:xrechnung_3.0 Version:2 Took:2933ms Errors:[] ErrorIDs: []
<?xml version="1.0" encoding="UTF-8"?>

<validation filename="ubl_incorrect_test.xml" datetime="2026-02-04 12:31:45">
  <xml>
    <info>
      <version>2</version>
      <profile>urn:cen.eu:en16931:2017#compliant#urn:xeinkauf.de:kosit:xrechnung_3.0</profile>
      <validator version="2.21.0"/>
      <rules>
        <fired>11</fired>
        <failed>0</failed>
      </rules>
      <duration unit="ms">2685</duration>
    </info>
    <summary status="valid"/>
  </xml>
  <summary status="valid"/>
</validation>

KoSIT Output

java -jar validator-1.6.0-standalone.jar -s validator-configuration-xrechnung_3.0.2_2025-07-10/scenarios.xml ubl_incorrect_test.xml 
KoSIT Validator version 1.6.0
Loading scenarios from  file:///xrechnung/validator-configuration-xrechnung_3.0.2_2025-07-10/scenarios.xml
Using repository  file:///xrechnung/validator-configuration-xrechnung_3.0.2_2025-07-10/

Loaded "Validator Configuration XRechnung 3.0.2" by Coordination Office for IT Standards (KoSIT) from 2025-07-09 
The following scenarios are available:
  * EN16931 XRechnung (UBL Invoice)
  * EN16931 XRechnung Extension (UBL Invoice)
  * EN16931 XRechnung CVD (UBL Invoice)
  * EN16931 XRechnung (UBL CreditNote)
  * EN16931 XRechnung CVD (UBL CreditNote)
  * EN16931 XRechnung (CII)
  * EN16931 XRechnung Extension (CII)
  * EN16931 XRechnung CVD (CII)
  * EN16931 (UBL Invoice)
  * EN16931 (UBL CreditNote)
  * EN16931 (CII)


Processing of 1 objects started
Processing of 1 objects completed in 299ms
Results:
-------------------------------------------------------------------------------------------------------------------------------------------------------
|File                                                       |Schema |Schematron|Acceptance|Error/Description                                           |
|/xrechnung/ubl_incorrect_test.xml        |   Y   |    N     |  REJECT  |[UBL-SR-53]- CompanyID (VAT Identifier) must be stated when |
|                                                           |       |          |          |providing the PartyTaxScheme/TaxScheme/ID.                  |
-------------------------------------------------------------------------------------------------------------------------------------------------------
Acceptable:  0  Rejected:  1


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