The progression system used by the game uses both XP earned as well as accomplishment based unlocks and on-mission sample gathering to help upgrade various options in their arsenal.
Along the way earning new Ranks, the player will unlock additional Primary Weapon options, Perk options, and cosmetic Outfits.
One of the best 4P co-op multiplayer game (local or online) I have played to date!
If you are a fan of the genre, just give it a try! :)