If you need to add some Sales header reference from Business Central to the

PEPPOL , following options can be used added to the informations your customer needs

<cac:AdditionalDocumentReference>

 <cbc:ID schemeID="AMH">Case-123456</cbc:ID> <cbc:DocumentTypeCode>130</cbc:DocumentTypeCode> </cac:AdditionalDocumentReference>

AMH – Case number
AEP – Project number
AKZ – Policy number insurance 
AEV – Metering point
ARR – Social security number
AWV – Phone number
BW – Batch/lot/package number
UAR – Serial number