A tier is a major waypoint in a character's level career. At every tier level the player has to choose between two different class options. The table below explains the tier numbers, the level you attain that tier, and the classes you can choose at that tier.

Tier Level Career Class Names
11 Recruit
25 Soldier Specialist
315 Commando Ranger Sapper Biotechnician
430 Grenadier Guardian Sniper Spy Demolitionist Engineer Medic Exobiologist