There are two types of outlet covers used on external outlets. The first type is for damp locations that covers the outlet when it's not in use:
And the second type is for wet locations that allow the outlet to be sealed while in use:
How is wet vs damp defined by NEC to differentiate when one cover is required over another?
NEC 406.8 Receptacles in Damp or Wet Locations: "A receptacle
installed outdoors in a location protected from the weather or in
other damp locations shall have an enclosure for the receptacle that
is weatherproof when the receptacle is covered (attachment plug cap
not inserted and receptacle covers closed).". And "receptacles
installed in a wet location shall have an enclosure that is
weatherproof whether or not the attachment plug cap is inserted."
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