Most but not all manual transmissions use a heavy weight oil. That is transmission oil. Most A/T use a much lighter transmission fluid, which is a highly refined oil, that is transmission fluid. You will sometines find a manual transmission which uses Trsnsmission fluid. Certain Ford Rangers is an example.
Automatic transmission oil is basically hydraulic oil for use in pumps. It is NOT recommended for use in any manual style transmission. All automatic transmissions in ATVs are a CVT type, and use no fluids. You should buy the proper gear oil for your ATV transmission as instructed in the owners manual.