PLUGIN_HANDLED_MAIN:
the function will still be called in other plugins, but it will not be forwarded to the gamedll (in case of a dllapi function) or the engine (in case of an engineapi function).
The client_command supports this return value for example. I think register_clcmd-ed commands don't (yet).
If you are building a module and using the AMXx forward system, use ET_STOP2 instead of ET_STOP as the exec type parameter to support the PLUGIN_HANDLED_MAIN return value.