you need to specify the DUNS number as the
external ID when setting up a partner in Trading Networks. I believe when
you select DUNS External ID Type, Trading Network expects the qualifier to
be 01, though the acual ID can be anything including a DUNS+4 code. These
should match the ISA Sender ID and qualifier. The GS ID can be anything.

What does above sentence mean in EDI processing through Trading network?

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)

Envelope Header
ISA00 00 019251750000 081234567890 0306271304U004010000014030P>~

Group Header GSPO9251750000123456789020030627130
41403X004010~

Transaction Segment
ST85001403001~

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.

https://www.amosoft.com/edi_docs/edi_850_x12_sample.html

HTH,
RMG