The "AE" horses appear in the program at the bottom of the list of horses entered in a race, usually in a shaded box with "AE" next to them. That means these horses are "Also Eligible". Which means that if only 10 horses are allowed in a race but more than 10 are entered, the horses who definately get in the race have satisfied various criteria, and the rest are on stand-by. If one of the definate horses scratches out of the race prior to a set deadline on the morning of the race, then an AE horse takes its place, in a set order. It's my understanding that the preference system and also the scratch time deadline can vary from track to track - someone please correct me if I'm wrong about that.
Owners need to understand this part of the game. It's a part that can be foggy for owners and bettors alike. It's tough for owners to plan to attend a race when their horse is "on the AE", but it's part of the drama! Today Jere talked to his owner at about 11:30 on the day of the race to discuss "plan B" since their horse on the AE that day did not get in. Some owners think they have until the time of the race to maybe get in if a horse scratches, but that's not the way it works in the United States.
Jere had a good time relating how in Saudia Arabia he had to have his horse ready up until the time of the race in case a horse scratched to let his horse in. Many things can happen between scratch time and the race - the state vet can scratch a horse for signs of lameness in the post parade, a horse can get loose and run off before the race, etc. But as he told his owner, as of 11am on the day of the race, if your horse doesn't scratch into the race, "The Party's Over, Done and Dusted. Time for plan B."