According to the 5010 837I IG, it is required that the GS06 be unique within a single interchange, but it is not required (though recommended) that it be unique within all transmissions across a period of time. Given this, I assume that even within a single day, it is possible for there to be duplicate GS06 control numbers/ST02 control numbers for different interchange transmissions. Since this could occur, why doesn't the 5010 999 ack contain a field to map the Interchange Control Number (ISA13) of the originating transaction? For example, interchange 12345 and 67890 are transmitted on the same day for the same Trading Partner for 5010 837I txns. Both transmissions contain GS06 13579 along with the same ST02s and a 999 is generated for both transmissions. How would the receiver of the 999 correctly link them back to the original transmission?
The GS is created before the ISA, and therefore we cannot reference the ISA within the GS.
From the standard document 12.6 Application Control Structures:
3.9.1 Functional Group Header and Trailer
“In order to provide sufficient discrimination for the acknowledgment process to operate reliably and to ensure that audit trails are unambiguous, the combination of Functional ID Code (GS01), Application Sender's ID (GS02), Application Receiver's ID (GS03), and Functional Group Control Numbers (GS06, GE02) shall by themselves be unique within a reasonably extended time frame whose boundaries shall be defined by trading partner agreement. Because at some point it may be necessary to reuse a sequence of control numbers, the Functional Group Date and Time may serve as an additional discriminant only to differentiate functional group identity over the longest possible time frame.”
The scenario described in RFI 1220 is a bad business practice which can overwhelm the ability for trading partners to operate reliably.