You must have completed the required steps in Extending an IDoc Type .

Your extended function module is addressed in partner profiles through a Process code . All partners, whose partner profiles contain this process code (and the corresponding message type) process the data of this extension in the system. If you want to avoid this with particular partners, then you must create a copy of this (originally not extended) function module and use a process code which refers to the copy.

For partner profile, please refer to the section:

Partner Profiles in the Standard Dialog