So, does being entered in a breed studbook actually mean anything for the stallions? I can't seem to find a areport listing entries, and I've bred and bought stallions that had it listed as an achievement. (Actually bought them to geld for showing, but hey, if this studbook thing makes them valuable, then maybe I'll wait). Thanks for any help answering this.

Nope! The stud book doesn't have any function in game.

What Lost said, and it also means they have a rating if P or higher in any trait

Thanks! I didn't think it did, but since I'm so new to the game it was worth asking.