A jacket was an article of clothing that could be worn for style or protection from the weather. A heavy jacket was also be called a coat. Both jackets and coats usually had pockets that are used to hold equipment.
While exploring the cold surface of the only planet in the Jinoteur system, Starfleet Intelligence agents Bridget McLellan and Cervantes Quinn wore heavy jackets. Quinn kept a flashlight in his jacket pocket. (VAN - Declassified novella: The Stars Look Down)
In 2370 Doctor Julian Bashir and Lieutenant Jadzia Dax had the option to wear Starfleet issued jackets or environmental bracelets when beaming down to the cold surface of Keltara (DS9 comic: "For the Glory of the Empire")
- The cover to the comic showed the team wearing jackets, but in the comic they opted for the bracelets.