In Battlefield 2142 every player can unlock and use different abilities. Any ability is for the player in general, not locked to a specific class. You can unlock them whenever you want to, but you can only unlock them in sequence; first the top one on the list then the one below and so on.
Aside of those abilities, each class (excluding Recon) has a basic ability such as Heal, Rearm and Repair (these abilities are respectively held by the Assault, Support, and Engineer classes). Improvements to those abilities can be unlocked; visit the unlocks page for more details. Each class also has a different NetBat ability; these abilities benefit the player's entire squad.