There are no classes. Six role tracks level from what you do: Infantry, Medic, Support, Recon, Pilot and Driver. Career, also called Wardog level, is the sum of your six role levels, so no XP is wasted. Career gates armour, helmets, backpacks, vests, ammo types, optics and pistols.
Season 1 XP is cumulative: 35,000 [S1] to reach level 10, 110,000 [S1] for level 15, 250,000 [S1] for level 20 and 1,000,000 [S1] for level 50. Levels 15 to 20 are the wall at 140,000 [S1] XP for five levels. BULKHEAD expects the median player to be around level 20 after 20 to 25 hours.
To level fast, do everything inside the hotzone, where XP doubles. For Career, level Support: building, repairing, refuelling, logistics, explosives and high-value target kills all feed it. For one deep track, commit early and do not start a second deep track after the first week.