In the year of YE 30, during the shift in government and military. It was found that a new set of ranks were needed to compensate for the split between the Navy and the Marines. These ranks, are considered current and correct as of YE 30, post-Kennewes Offensive.
Marines are personnel who typically operate in the field and/or serve as armored infantry. The Marine's job is to get the dirty work done with their hands, not with their ships. Those wearing the red bars of a Marine, wear the standard green uniform.
Officers of the Marine Corps are mainly used to coordinate large masses of marines when available. In other situations, they're used to regulate the different Corps, including their own corps.
Warrant and Joint personnel in the Marines are people who command the large groups of marines on the ships and on the ground.
Enlisted marines are the bulk of the Marine Corps who fight on the ground, and get the killing done.
Naval personnel often operate within ships, or in position which wouldn't require them to drop down into the field like Marines. They are typically people who arn't in the chaos of planetary warfare, but are used to the chaos of space battles. Those wearing the black bars of a Naval, wear the blue uniform.
Officers in the navy are usually bigwigs that command fleets, or single ships with a full list of crews or a single crew under their command.
Warrant and Joint officers in the Navy make up the bridge crew of any ship in a Star Military Fleet. They also make up the higher ranked members of the Public Relations Corps.
Enlisted soldiers in the Navy are usually crew hands. Medics, Engineers, Cooks, people who arn't in the bridge room. They keep the ship and the people on it running at full capacity.