Crew rest facilities are certified by the FAA. If an airline wants to use a cabin seat instead of a dedicated rest facility that specific seat must be approved. Rest facilities are approved as a class 1,2 or 3 facility. Generally on a long haul aircraft the rest seat will also include a curtain to get a higher rating which permits a longer crew day.

a. Class 1 Rest Facility. Means a bunk or other surface that allows for a flat sleeping position and is located separate from both the flight deck and passenger cabin in an area that is temperature-controlled, allows the flightcrew member to control light, and provides isolation from noise and disturbance (§ 117.3, "sound" definition Society of Automotive Engineers (SAE) Aerospace Recommend Practice (ARP) 4101/3 and "horizontal flat" definitions, SAE ARP
b. Class 2 Rest Facility. Means a seat in an aircraft cabin that allows for a flat or near flat sleeping position; is separated from passengers by a minimum of a curtain to provide darkness and some sound mitigation; and is reasonably free from disturbance by passengers or flightcrew members (§ 117.3).
c. Class 3 Rest Facility. Means a seat in an aircraft cabin or flight deck that reclines at least 40 degrees and provides leg and foot support (§ 117.3).