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. ()

22nd century

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. ()

23rd century

Wraparound tunic

Late 2250s-Early 2270s 

Captains had a choice of relaxed clothing to wear when on duty, in the form of a wraparound tunic. In 2259, Christopher Pike wore a green, leathery tunic with high shoulders; unlike future versions, it was worn with a standard Starfleet delta badge on the chest in addition to the delta clasp at the waist. ()

James T. Kirk favored two green variations in the 2260s. Charles Evans also wore a beige version of this tunic, though where it originated from is unknown. ()

The captain of the  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  found the tunic in his quarters and wore it himself. ()

Mid 2270s 

This wraparound tunic was worn by Lieutenant Commander Hikaru Sulu shortly before the  departed drydock to confront V'ger and during Admiral Kirk's briefing to the entire crew. This tunic had short sleeves and was worn over a black undershirt. Epaulets denoting rank were found on the tunic. Pants matching the color of the jacket were also worn. A metal Starfleet emblem found near the waist on the jacket was used to close it over the undershirt. ()

Bomber jacket  

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

24th century

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 . (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. ()

Neelix's duty attire 
During Neelix's time joining the crew of the  as morale officer and cook, he wore a number of Talaxian outfits while on duty and serving in an official capacity. (, et al.)

Despite Neelix's interest, from his first day aboard Voyager, Tuvok indicated the ship would not replicate him a Starfleet uniform. ()

In an alternate timeline, during the Year of Hell, Neelix was given a field commission and wore an operations division uniform. ()

Appendix

Background information 
Troi's low-cut outfits were a throwback to the costumes of . () The costumes were all cut differently and some were lower than others, exposing more of Troi actress Marina Sirtis' cleavage to the camera. (The Fifty-Year Mission: The Next 25 Years, p. 102)

In the "Stardate Revisited" documentary included with the TNG Season 1 Blu-ray collection, Marina Sirtis described the "gray unitard" outfits from the first season as being made of denim ("...and not the stretchy kind... like wearing skin-tight jeans up to your neck"). Those costumes were an attempt to design an outfit for Troi that would be more flattering than the "skant" uniform, which she wears in TNG pilot . "So they came out with the ugly gray spacesuit," Sirtis recalled, "and they put a belt in a lighter-colored fabric exactly where my fat was." (The Fifty-Year Mission: The Next 25 Years, p. 101) In essence, Sirtis hated those grey costumes. (Cinefantastique, Vol. 23, No. 2/3, p. 77)

At the start of Season 2, Troi's maroon-colored costume was designed by then-new TNG Costume Designer Durinda Rice Wood. Marina Sirtis found it "much more flattering" than the grey unitards. (Cinefantastique, Vol. 23, No. 2/3, p. 77) Costume Designer Robert Blackman, who succeeded Wood, didn't approve of the jumpsuit, however. "I kept wondering why her uniform was a slightly different color, and more vulnerable-looking," he said. "I was told that it was not a uniform – they are day clothes. I said it was not happening for me." ()

Marina Sirtis cited Robert Blackman as the originator of Troi's "wardrobe" of casual attire. "He came up with the idea that if Troi's not wearing a Starfleet uniform, then she should have more than one outfit to wear," the actress noted. (Cinefantastique, Vol. 23, No. 2/3, p. 77)

Troi's blue-green dress was designed by Blackman, inspired by his opinion that she needed to look more vulnerable and softer than the maroon outfit made her seem. "So I came up with a fabric that's a wonderful shade of blue-green," he commented, "with the asymmetrical neckline, a dress and pants." According to Blackman, both he and Marina Sirtis were happy with the dress, as were most other people. Blackman thought the dress gave Troi a softer image and made her appear less officious, giving the impression she was more accessible. () However, Sirtis described the garment as, "That green dress, the one you wanted to straighten out the neckline on all the time." She went on to say, "I wasn't crazy about the dress, because you had to take the whole thing off to go to the bathroom. But the underwear was fabulous. I had to wear a corset like a merry widow, and then we had what I like to call the industrial-strength, Starfleet-regulation brassiere. This became the standard uniform for every woman on Star Trek, and that's because the women saw me as me and then they saw me as Troi, and they went, 'I want a bra like that,' because it adds inches where there really are none. It is kind of depressing at the end of the day when you take it off." (The Fifty-Year Mission: The Next 25 Years, p. 102)

Aside from wearing the skant uniform during "Encounter at Farpoint", Marina Sirtis was always shown wearing casual duty attire except in one instance before the sixth season episode  – in a fictional future presented in the Season 4 installment . After "Chain of Command", Sirtis wore the unitard costumes only thirteen more times, predominantly in post-duty-shift settings, and always appeared in a standard Starfleet uniform as well. Those subsequent episodes were , , , , , , , , and the series finale, .

Starfleet uniforms