Beginning in Season 4 of "Star Trek: Enterprise", science officer T'Pol received a Starfleet commission granting her the rank of Commander. However, afterwards she continued to wear civilian Vulcan clothing. This attire choice made sense when she was a member of the Vulcan High Command (during the first two seasons) and, later, a civilian (third season). As a member of Starfleet, though, she would be subject to the same dress code as other crewmembers.

Out-of-universe, her attire was obviously given the "7 of 9" treatment (attractive actress in tight-fitting bodysuit). I'm curious about the in-universe explanation, though, and wanted to know if it was ever addressed within the show or its spin-off materials.

While in Starfleet, why was T'Pol exempted from Starfleet uniform protocol?