What is a MIG subsystem and can anyone give an example of creating one?

Question

I've been looking at Mac OS Architecture and I found something refereed to as MIG. I think it is used for inter process communications but I'd like to find out more.

Can someone tell me more about MIG (and what it stands for) ?

Here is a strucure of a 'defs' file:

subsystem example 36270;

#include <mach/std_types.defs>

routine mig_test(
    server : mach_port_t;
out test : int);