Okay, from my tests (with the web page mentioned in an earlier test) I
think Marcus' assumptions were right. In other words the DC only
looks at the two least significant bits

(i.e. %0000000000000011)
^^

The zero'th bit indicates whether the file is copy protected or not
(0 = copyable, 1=protected)

and the first bit indicates file type
(0 = data, 1=game)

therefore the possible types are:

0 (%00) = Copyable data
1 (%01) = Protected data
2 (%10) = Copyable game
3 (%11) = Protected game

I tested up to file type 6, and as far as I can tell they just cycle
round on a modulus of 4, indicating that only the two bits I mentioned
are actually used (so type 4 = type 0 ; type 5 = type 1 ; type 6 =
type 2 ; etc)