At the time of writing, there's no legally required standard in the US. In theory, anyone can make armor for your elbows or back out of whatever material they like. In practice, most manufacturers of high-quality motorcycle jackets follow the European EN or CE standards. Armor is rated CE Level 1 or CE Level 2 (sometimes just called Level 1 and Level 2). The ratings tell you the maximum impact force transmitted to the rider in kiloNewtons (kN), which is all a bit technical. Suffice it to say that Level 2 provides more protection than Level 1. You often find that elbow protectors are Level 1 and back protectors are Level 2.