Think of it like Naval uniforms for officers. Line officers are denoted with a star over their rank stripes. Line officers can do numerous tasks, but their main focus is command-oriented. Specialties (docs, some supply, chaplains, legal, etc.) have different uniform insignias over their rank stripes. Red shirts woud be like line officers. Because it never appears that you have enlisted on the bridge (unlike the USN), you would have either line officers or a specialty (science, engineering). The only other things redshirts get to do is die, and those are the enlisted.