Yes. The ABLB is the top leaderboard.
These will be the strongest horses in the game.
Breed leaderboards are also good to be on.
If your horse is on the ABLB it will also be on its breed LB.
If youre breeding horses for AD ratings, you will focus mainly on the eventing LBs.
Depends on which board the horse is on. I wouldnt really think much. I aim to get my horses top 10 on their breed boards. So 17 would be good, but not excellent.
As far as breeding goes, some breeds arent on the ABLB. Like WBs. So i use the studs in the top 10 of the WBELB. (Warmblood eventing leaderboard).
If you have ablb studs in your breed available, use them. If not, use the top studs in the breed eveneting board. :)