From the "FIM" Rule Book, page #36, 2.11 Protective Clothing and Helmets,
2.11.1 Riders and passengers must wear a "complete leather" suit with additional leather padding or other protection on the principal contact points, knees, shoulders, hips etc
2.11.3 Riders must wear "leather" gloves and boots, which "with the suit" provide "complete" coverage from the neck down.
2.11.4 Leather substitute materials may be worn, provided thay have been checked by the Chief Technical Steward.
I sent a email to Tech@bonnevillespeedtrials.com ten days ago seeking clarification on this, to date no reply