You used to apply for a general provisional license at 16, then as you reached each age milestone it became valid for the relavent categories. As you passed a category test (eg 'F' for agricultural tractor) your licence became a full licence for that, (and in the old days changed colour from green to pink when you sent it off!) but remained provisional for those you hadn't passed. I believe the same system still holds.
Just to complicate things a bit, by passing 'B' (a car test) you also pass some other categories, including 'F'.