Following discussion with trainers, the BHA and Weatherbys have clarified that, for the purposes of racecard definition, a rig is classified as a horse that has no external testicles (i.e.) both testicles are undescended. Such horses are unlikely to be fertile. Should both undescended testicles be removed, the horse should be classified as a gelding.
Horses which have one undescended testicle and one external testicle are likely to be fertile, and for the purposes of racecard definition should be classified as colts. Should the undescended testicle be removed, the passport should be clearly endorsed by the veterinary surgeon to state that this has occurred (i.e. that the horse has one external testicle only.) In these instances, the passport should be sent to Weatherbys for endorsement. Should the passport be misplaced in the future, there will be a record that the horse has one external testicle only and a replacement passport would be marked as such. These horses should continue to be classified as colts.