In the beginning of the third season the uniforms were redone from the old "jumpsuit" style uniforms to snazzy, shoulder-padded, custom-fitted uniforms. With a back zip. The theory is that if you were an important character, you got somebody in your trailer to help you dress, and zip you up.
Unimportant characters got a one-size-maybe-fits-some-people uniform that zipped up the front. Because who is going to shill out $6,000 on Ensign NoLines?
So the position of the zip on a uniform became a quick reference to how important a character was.