When you maintain a communication system in SAP C4C for integration with an SAP backend system (ERP, S/4), you flag the "SAP Business Suite" field and then maintain the "Business Instances".
The terminology has always confused me, but I just came across a pretty old blog by SAP that explains these fields under a somewhat unintuitive title: Communication System Setup and Message Header Hand... - SAP Community.
In short:
- System Instance ID - you choose this ID in C4C, same as your communication system ID.
- Business System ID - backend system ID used in web service based integrations (e.g. transaction DRFIMG, define business system)
- IDoc Logical System ID - backend system ID used in IDOC integrations (as maintained in BD64 distribution model)
At the end of the day, C4C doesn't care what you have maintained in your backend system, it only cares about the payload. So you can even get away with simply maintaining the correct ID in the corresponding iflow's mapping, though this would be bad practice.
Which of these fields C4C cares about, is derived from the incoming payload, RecipientParty, InternalID, SchemeID: e.g. if schemeID is "LocalSystemID", then the InternalID corresponds the IDoc Logical System ID. Mileage seems to vary if it is web-service based, examples I found (e.g. Exchange Rate replication) leave the schemeID empty.
