The Marines were initially kept under the umbrella of the Army’s command structure, they weren’t officially separated and formed as their own division until the end of the First Tevarin War. The UPE felt that they needed a consolidated special operations force that commanded the best of both the Army and the Navy. Their focus and primary application became planetary invasion. Marine units are specialized to be weapons of warfare, nothing more. They don’t handle diplomatic escorts or pull guard duty, they dust things. When the Imperator doesn’t need them, the Marines sequester themselves to their planet-bases on Corin (Kilian system) and train for the next conflict that will require their services.
Marine-Candidates are selected from the general pool of Army and Navy recruits. The criterion for selection is unknown and inconsistent at best. Some speculate that it is intentional to cement the fact that you don’t apply to be a Marine.