Hello,
If I am not wrong you are talking about below scenarios
These scenarios need not wait for each other( independent of each other)
In case of outbound EDI interfaces, EDI 997 (if partner is capable of sending one) will be helpful in updating the status of the Outbound IDoc.
In Inbound EDI scenarios, 997 is generated at adapter level as s0on as message reaches PI successfully, 997 is sent back to partners if requested.
To understand more on acknowledgements please have a look at below blog
Functional Acknowledgement (Inbound and Outbound FA 997)- for Beginners
(P.S.- ACK 855 is an interface on its own (needs mapping from EDI to SAP (Idoc, rfc, file..))
Regards,
Pooja