Casual duty attire

Casual duty attire was informal clothing worn instead of a standard uniform while on duty.

On some Starfleet vessels, certain crew members were granted permission, at the captain's discretion, to wear casual duty attire while serving on board. This diversion from the standard uniform code was typically granted to senior officers in specialized positions. 

T'Pol's uniform
In 2153, science officer T'Pol resigned from the Vulcan High Command. As she was no longer permitted to wear her former uniform as a civilian, she opted to don brightly colored, tight-fitting jumpsuits while on duty. After T'Pol accepted a commission to commander in 2154, her civilian jumpsuits were modified to include division coloring, rank insignia, and other details consistent with standard uniforms of the period. T'Pol continued to wear variations of her civilian jumpsuits until at least 2161. 

Mid 2260s-Early 2270s
Captains had a choice of relaxed clothing to wear when on duty, in the form of a wraparound tunic. James T. Kirk favored two green variations. Charles Evans also wore a beige version of this tunic, though where it originated from is unknown. 

The captain of the USS Defiant (NCC-1764) also had a green tunic when the ship disappeared through a spatial rift in 2268, to reemerge in the mirror universe in the year 2155, where $𝜇$ found the tunic in his quarters and wore it himself. 

Bomber jacket
In keeping with the Starfleet practice of allowing officers of command rank to wear a less formal variant of the uniform, starting in the late 2270s command officers were given the option of wearing a suede leather bomber jacket version of the uniform. This was worn over the department color turtle neck. 

Deanna Troi's duty attire
Between 2364 and 2370, Deanna Troi frequently wore casual, nonuniform attire while on duty as ship's counselor, as an alternative to a standard Starfleet uniform. She primarily favored unitards in a variety of colors. ( – TNG Season 6) Occasionally, she wore a turquoise variant with a long skirt. 

While serving under Edward Jellico in 2369, she was ordered to wear a standard uniform while on duty. After Jellico's order, she wore a traditional uniform almost exclusively during the rest of her posting aboard the USS Enterprise (NCC-1701-D). (TNG Season 6 – )

Captain's variant
By 2368, Starfleet had a uniform variant available to commanding officers in service aboard starships, consisting of a red command division suede jacket featuring banded cuffs and either leather or fabric black shoulders. Less formal than the standard duty uniform, this "captain's jacket" was a more casual dress option, with trousers reminiscent of the previous Starfleet uniform.