Per your request.
From the USEF rule book:
EQ 106.6. When showing in the Hunter/Jumping Seat Equitation section it is recommended that riders use traditional stainless steel stirrup irons that promote proper position of the foot in the iron as well as a correct leg position. It is further recommended that riders use stirrup irons that allow judges a clear and unobstructed view of the position of the foot in the stirrup. Judges may not eliminate a rider for using a particular style of stirrup iron. (Exception: see EQ101.4)