A station that hears this may adopt the constraints in it, which makes this element the mechanism by which a device entering a country learns what it is allowed to transmit. It is advisory and unauthenticated — nothing stops an AP claiming to be somewhere it is not.
Subfields
Country String — Two characters of ISO country code, then one saying whether the limits that follow are for indoor use, outdoor use, or both.
Channel constraint — First channel number, how many channels follow it, and the maximum transmit power permitted across them.