I believe you have some EDI knowledge and the above statements is holds for EDI X12 transactions processing in TN.
DUNS and DUNS+4 is related to EDI qualifiers in the ISA (starting Envelope Header Segment) which comes in the ISA05 (DUNS Qualifier-01)/ISA06 (actual DUNS value)
ISA00 00 019251750000 081234567890 0306271304U004010000014030P>~
Group Header GSPO9251750000123456789020030627130
Please take some time and google it related to EDI X12 samples related to Qualifiers/Segment structures etc…for more basic information of EDI and you can correlate it in the TN Partner configuration and setup the External ID’s associated to ISA/GS envelopes…But if you don’t setup correct in the TN for ID’s then you will see the Invalid Sender/Receiver recognition errors in the Transactions page during it’s processing to avoid the default rule triggering.