North American AM station classes

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Information derived from https://www.fcc.gov/media/radio/am-clear-regional-local-channels. This table has been produced for convenience, embedding, and reference purposes.

Station class	Time/Daypart	Power restriction
Class A (clear channel)	Unlimited, round-the-clock operation	Not less than 10 kW, nor more than 50 kW
Class B	Unlimited, round-the-clock operation	Not less than 0.250 kW (with exceptions), nor more than than 50 kW
Class C	Unlimited, round-the-clock operation	Not less than 0.250 kW (exception: see note below table), nor more than 1 kW
Class D	See Class D operations below table	Not less than 0.250 kW, nor more than 50 kW daytime; ~0.250 kW or less nighttime
Travelers' Information Stations	Unlimited, round-the-clock operation	Not more than 0.010 kW. On 530 KHz only.

Regarding Class C stations: prior to a date which I can't quite fully determine, class C stations were licensed to operate at 0.100 kW or less. Stations which were licensed for 0.100 kW or less appear to be grandfathered, and do not have to increase power or alter their operations in any way.

Class D operations

Relevant document: "In The Matter of Unlimited-time Operation by Existing AM Daytime-Only Broadcast Stations; Discontinuance of Authorization of Addtional Daytime-Only Stations; Minimum Power of Class III Stations" on KONAT.xyz (PDF; mirroring the original on FCC.gov)

Class D stations licensed prior to November 18, 1987 were daytime-only, meaning they operated between dawn and dusk. After that date, new Class D licenses have been for round-the-clock operation, albeit with some having provisions for reducing power overnight, at critical hours (the hours of dawn and dusk). The current regulations for Class D stations closely mirror the FCC's regulations for Class B stations; the FCC themselves even will refuse to issue Class D licenses (unless the station is downgrading nighttime operations to explicitly meet Class D regulations), and will instead issue a Class B license.

In spite of that, daytime-only Class D stations licensed prior to 1987 may continue operating in a daytime-only capacity.