Basically for the D2 scripts they do one of three things:
(a) draft a ship: here they basically specify a hull class range, friend or foe, and a bpv range. The BPV range can either be expressed absolutely (draft a ship between BPVs X and Y) or as a ratio of the drafting player's BPV (e.g. .6 to 1.2 times the BPV of the drafter). The difficulty modifier from the MissionMatching.gf file also gets factored into the BPV range. It can then draft or generate any ship in the shiplist that matches the parameters given, EXCEPT ships marked as "special" in the shiplist. So nearly anything you add to the shiplist has a chance of being drafted except things you classify as special.
(b) create a ship in a specified class range and centered around a specific BPV value: pretty similar to the above, except that the ship is never drawn from the D2, it's simply a generated NPC for that mission
(c) create a specific ship, e.g. "generate a F-CC"
dave