Types vs subtypes

["Alex Angas" <wgd@adelaide.on.net>]

I've been given an Ada project to work on and I'm trying to work out what
the difference between using a type or subtype is. Why is myarray an array
of st, instead of just making it an array of t in the following declaration?

 type enum is ( red, green, blue ) ;
 type t is array( enum ) of natural ;
 subtype st is t;
 type myarray is array( enum ) of st;

They both seem to compile the same. What am I missing?

Thanks for your help,
Alex.