No set classes. You buy the equipment for the role you want, and the gun in your hands decides which track earns XP. A bigger backpack lets you carry kit for two roles.
Assault rifles and pistols feed Infantry. SMGs feed Medic. Shotguns, LMGs and rocket launchers feed Support. Marksman rifles, sniper rifles and the compound bow feed Recon. Spotting feeds Recon, healing others feeds Medic, building and repairs feed Support, driving feeds Driver and flying feeds Pilot.
Not sure about an item? Hover it in the vendor or your inventory and it shows which track it rewards.