Difference between executing a command and calling a subsystem?

[Dan Kaminsky]

>Subsystems must be pre-defined in sshd_config. They are intended more as
>a sub-protocol extension mechanism (e.g. sftp) than as a user-visible
>way to execute commands.
>
>As far as execution of commands go, there is very little difference in
>terms of code executed between a defined subsystem and "ssh somehost
>command"
>  
>
Isn't their execution environment much more strictly defined, i.e. 
pathing issues aren't a problem like with scp, and it's fair game to 
specify _protocols_ rather than _implementations_?

Is the user's shell still invoked to execute subsystems?

--Dan