The 21st Marine Division is a Marine Infantry Division Tasked to Locate close with and destroy the enemy by fire and maneuver and repel the enemy's assault by fire and close combat. The 21st maintains 2 Infantry Regiments currently with 1 Artillery Regiment along with several specialized battalions like combat engineers, a reconnaissance battalion, scout snipers, light armored reconnaissance battalions, and so on. 

The 2nd MAW Conducts air operations in support of the Marine forces including Offensive air support, anti-air warfare, assault support, aerial reconnaissance, electronic warfare, and control of aircraft and missiles. As a collateral function, the MAW may participate as an integral component of naval aviation in the execution of such other Navy functions as the Fleet Commander may direct. To join the 2nd MAW you’ll have to pass an SOI and after that, if you do join you will automatically be placed in the Marine Air Control Group as a ground crew member in either air traffic control or so on. After that, you attend pilot school unless you wanna be a part of the ground crew.

The 506ths role is to provide armor-protected firepower and shock action in support of marine infantry forces. How to join these tankers is currently unavailable. 

7th Orbital Drop Shock Trooper Battalion is special operations capable organization of the UNSC Marine Corps that specialize in orbital-dropped shock infantry and special warfare tactics.