now i may not be understanding your question exactly but i will try to cover all the bases here:
when you configure a party in biztalk for EDI purposes you need to specify the Sender ID, the Qualifer and the Sender control. In the EDI world, thats the ISA05,ISA06 and GS02 for the sender and ISA07, ISA08 and GS03 for the receiver respectivly.
SO….. hopefully, this next part answers your question….
any XML to EDI (or vice versa) translator, be it the HIPAA accelerator or Covast, will take this party information and map it to the envelope. They will also use the party information to validate that this person sending the file is in fact a trading partner using proper IDs.
And thats the significance of parties in biztalk and EDI. hope that helps get you on the right track.