It may be a standard 120 volt 20 amp outlet, rather than the much more common 120 volt 15 amp outlet.
Common USA practice is to connect multiple 15 amp outlets to 15 amp or to 20 amp circuits.
If higher loading appliances are to be used, then 20 amp outlets on 20 amp circuits.
The very common 15 amp plugs will fit 15 amp outlets or 20 amp outlets.
The less common 20amp plugs will only fit the 20 amp outlets, a 15 amp outlet will reject the 20 amp plug.
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