A uniform code is a regulation governing the proper manner in which a uniform must be worn.
In 2371, Lieutenant Tuvok was given the responsibility of training four Maquis crewmembers in Starfleet rules and procedures. During the initial briefing, he noted that scuffed boots, Bajoran earrings, elaborate headbands and jewelry, even if hidden from view, were forbidden under the Starfleet dress code. (VOY: "Learning Curve")
While on duty, Major Kira Nerys often wore a bracelet given to her by Tekeny Ghemor, though she admitted that doing so was not permitted by the Bajoran Militia uniform code. (DS9: "Ties of Blood and Water")