In most prize lists that I have read, the Adults have height restrictions on the rider. I know for Hits, the Adult's (hunters) can not cross enter into any classes above 3'3, and the jumpers can't cross enter into anything above 1.15m. the prize lists usually say what heights the Adults can cross enter into. NOW some shows have low a/o hunters set at 3'3, so I'm not sure about that one. Hopefully someone else can chime in with definite answers..
I think it varies by zone, but I could be wrong...
Yes, Adult Amateur specs are set at the zone level and vary widely. Two examples are listed below.
Adult Amateur Horses may be shown by a different rider in any other division or class at the same competition. Riders may cross enter between the Adult Hunter and the Amateur Owner division if the rider owns the horse he or she is showing in the Adult Amateur division. Riders can cross enter between the Amateur Owner 3'3" and 3'6", but horses may not cross enter between the Adult Amateur and Amateur Owner Hunter sections at the same show.
Adult Amateur Hunter:
To be ridden by an Amateur in possession of a valid Amateur Card. A rider in the Adult Amateur Division is not eligible to compete in any class where fence heights exceed 3'3" with the exception of the Children's/Adult Jumpers and the USHJA National Hunter Classic. .. Riders who compete in the 3’3Amateur Owner Hunter classes must show a horse they own in Adult Amateur Hunter classes if they compete in both at the same show."
And there are restrictions at the A/O level too. But these are accross the board, national level specs.
This year an A/O rider can show one horse that they do not own in non A/O classes.
". and a maximum of one (1) horse not owned by the rider, or his/her family, in jumper sections and a maximum of one (1) horse not owned by the rider or his/her family in hunter sections, in classes not restricted to amateur owners"
Cross entering is specifically mentioned in the National A/O rules.
"Riders may cross enter between the Adult Amateur (if Zone Specifications permit), Amateur Owner 3’3” and/or the Amateur Owner 3’6” divisions. Horses may not cross
enter between the divisions"