Richard built the fixture with a predefined number of attributes. This way if the functionality is extended in the future, a user's show file will not be affected.
If you can imagine using a moving light in mode 1 and then later wanted to use the same fixture in mode 2 then you would need to reprogram your show if the mode changed the channel assignments.
Reserved is like a placeholder so that any new versions of Catalyst will still work identically (or better) on older programmed shows.