Publish & Subscribe interfaces:
Allow you to start one or more (multiple) additional operations when a particular event is triggered. They do not influence the standard R/3 program in any way.
Multiple operations do not interfere with each other.
Add-on components can only import data.
Possible uses: Additional checks (authorizations, existing duplicates, and so on)
Process interfaces:
In contrast to Publish & Subscribe interfaces, data exchange takes place in both directions with process interfaces. This influences the number of additions that can be attached to the interface.
When an event is triggered, a process in the standard program can only be replaced by a single external process using the process interface.
If you are using an add on from an SAP partner that uses a process interface, this enhancement is processed at runtime. If you choose to use this same process interface for one of your own developments, the partner enhancement is dismissed and your own enhancement is processed at runtime instead.