Service members in pay grades E-1 through E-3 are usually either in some kind of training status or on their initial assignment. The training includes the basic training phase where recruits are immersed in military culture and values and are taught the core skills required by their service component.

Leadership responsibility significantly increases in the mid-level enlisted ranks. This responsibility is given formal recognition by use of the terms noncommissioned officer and petty officer. A Marine corporal is considered NCO rank. The Navy NCO equivalent, petty officer, is achieved at the rank of petty officer third class.

Senior Non-Commissioned Officers often hold senior leadership positions (are in Command). In the Navy this is usually designated by a Badge worn on the left breast as well as the title "Command" before their rank. The FMF has differently named ranks for those positions (right hand side).

Only one person at a time can hold the rank of Master Chief Petty Officer of the Navy or Sergeant Major of the Marine Corps. He will be the senior enlisted advisor to the Chief of Naval Operations or the Commander Fleet Marine Forces (respectively).