Can someone explain to me the original purpose of the DS9 uniforms in-universe? Like under what circumstances they were to be warn as opposed to the TNG uniforms? Sisko, O'Brien, and others where the DS9 uniforms while on the station and the Defiant, but not on Earth or while on the Enterprise. Were the DS9 uniforms meant as the uniforms for non-ship-based assignments? If so, was it ever explained in any Trek extraneous material why they were changed to shipboard uniforms (as seen in Generations and Voyager)?