Experience Points are one of the main things in Crossfire. These enable you to rank up, unlock weapons, GP, and many other items.
Some modes like Mutation Mode, H.M, and H.M.X are special, because after mutants are killed, you can collect their EXP by holding the "E" button
Elimination Modes like Alaska can give you a ton of experience points. Each kill also matters on experience. Headshots for example may worth like 20 or 15 Experience depending on the type of the game mode.