From The Union Fleet Liaison
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Attached to the ships of the Union Navy, the Raiders serve as defenders and boarding parties in ship-to-ship combat. As such, they are trained in close-quarters combat, and are adept in the use of the sword and pistol. Attempting to attack a Union ship aboard a Megacorp Boarding torpedo is generally viewed as a death sentence.
When needed, the Raiders can be deployed in planetside operations as light infantry.
When not deployed in combat, the Marines serve as the ship's crew.
The requirements of technical know-how and the ability to fight in close quarters attracted many of the Union's Far'Wd soldiers to the Naval Infantry Corps.