Ranks are obtained as you earn experience points by battling bosses and fighting aliens in PvE. (Note that playing PvP does not grant any form of experience.) As you rank up, you will unlock new weapons, armor, and backpacks that can be purchased at the store. Your rank may influence how you are perceived by other players within a game, but it is important to note that ranks are not a definite indication of one's skill level.
As a general trend, lower ranks donate a newer player. Ranks 6-10 are considered mid-game players, who may carry weapons of varying strength and upgrade levels depending on personal choice. Ranks 10 and up are held as battle-hardened veterans, but it is not uncommon to see players with developed armories before Rank 12.
Aside from Rank 9 being identified as "Sergeant Major (SGM)", the remaining ranks have not been officially named by Freyr. The current names are based on the current U.S. Army, Marine, and Navy ranks. Ranks E-1 to E-9 are known as "Enlisted" or "Non-commissioned" officers, while Ranks O-1 to O-3 refer to "Commissioned Officers". Please note that most of the ranks listed here are purely conjectural. These names will not be replaced until they are given official names.