It is possible for an executable to actually contain multiple services. In such a case, you’d have a ServiceInstaller that represents each service, but a single ServiceProcessInstaller, that represents the executable.
Each service is implemented (in .NET) as a different class. But the multiple services (if they’re all started) would share an AppDomain (I believe). It’s probably quite rare for you to need such a setup, and in most cases, there’s a 1-1 mapping between executables and services (so you’d only have a single ServiceInstaller and ServiceProcessInstaller in the executable).