Most tracks send their scratches and changes to us multiple times. We typically get early scratches which are posted when wagering becomes available for the day, then late scratches usually about an hour prior to post for race 1. On days when there are potential weather issues, sometimes tracks delay sending these changes to make a decision about the day's races (such as taking races off turf or even a cancellation).
Common wording for scratches are as follows:
MTO-main track only. The horse will scratch if the race stays on the turf and will run if the race moves to the main track (dirt)
AE: Also eligible. This horse is also eligible to run in this race, provided a spot becomes available. A horse will need to scratch for any also-eligible to draw into the field and be allowed to run. Also-eligible go in post position order.
PrivVet: the horse was scratched by a private vet, their personal vet has certified the horse has an illness or condition that prevents it from racing
RegVet: Regulatory Vet. The track veterinarian has ordered the horse scratched due to some soundness or lameness issue.