EMC's Conversation w/ Motion Controllers

What's the nature of the conversation between EMC and a servo motion
control card? Can EMC be reasonably viewed as a black box with g-code
and encoder signals going in and strings on some alphabet going out
to a motion control card? But what alphabet and what strings? What
kind of interface exists between EMC and the controller? I suppose
different controllers accept different sets of commands, so EMC must
have a place where interface code can be grafted on--right?

Thanks in advance.

Jack