Different clients in one app

Depending on your specific requirements, this can easily done using groups, record security and objects permissions. Or, not at all.

I concur with Stefan's remarks. The different clients are just different roles they have in the application.
Based on their group membership they have different functionality on the User Interfaces and see different data sets.
Easy to do.

The only challenge I see is: if you have to apply this to an existent application and this topic was not in view when the application was build.