Armbands with medical cards are required for eventing in both the stadium and cross country phases, as well as any warm up over fences. They are generally required for XC schooling. Please refer to USEF rule book, section EV113, subsection 3 "Medical Cards" which can be found here.
These armbands are also a good idea for trail rides and endurance rides. And when not on your arm they're good "stuff" labels too - especially handy when traveling for shows.
They are clear plastic pockets with black elastic bands and velcro closure. Please email me if you're interested. I accept PayPal. Shipping to Canada is an additional $3.