Members of the Special Forces wear civilian clothing when off duty, or on duty in situations where they are to come into contact with the civilian population of a planet, as this aids the Operator's ability to blend in with his/her environment. Non-regulation hairstyles and facial hair are encouraged for the same reason.
While aboard a vessel, an Operator will wear a plain black jumpsuit or fatigues that may bear no rank, name or unit patch (depending on assignment). For formal occasions or operations of a covert nature aboard a Star Fleet vessel or outpost, Operators wear the uniform of the department they originally came from. This bears their rank and medals, but does not indicate the ship/station to which the Operator is assigned. For all intents and purposes, the Operator outwardly appears as a generic officer.
Infiltration Suit - Special Forces exclusive