The game will feature a vast set of skill trees comprising, according to Matt Higby, thousands of certifications. Skills are said to affect everything in the game, including weapons, attachments, vehicle training, upgrades, gameplay features and more. There will be so many skills that you could "play for a whole year, and still not max out something like a vehicle."
Each weapon, vehicle and class will have their own extensive set of skills. There is no restriction on the number of certifications available to a player (once they reach the maximum battlerank).
We're assuming EXP will be awarded, as before, for kills, hacks etc as well as successfully carrying out mission. Matt Higby has also confirmed that EXP will be proportionally awarded for kill assists, more so for squad assists. This will be calculated based on the percentage of damage you did and the time since dealing the damage. Support work, such as healing and repairing, will be rewarded too.
Skills are acquired using a combined online/offline training system. In order to unlock a skill, you must earn a Battle Rank by taking an active part in the game. Once a skill is unlocked, it can be trained after a period of real-world time has elapsed. You do not need to be online for this time period.
This system has been likened to that used in EVE Online.
The speed at which your skill certifications are trained increases based on your activity.
Battleranks will return and the current cap is level 20 although this is not set in stone. At the maximum battlerank, the entire skill tree will be available to you.
Higby has clarified the relationship between battle rank and certs. Certs are unlocked based on battle rank, but do not directly correlate. Advancing to Battle Rank 10 does not mean you get a 'level 10' in all skills.
You can switch to any role you like, but unless you have put time into advancing the certifications making up a role, you won't be very effective. In addition, buffs for some advanced roles are only unlocked after advancing along an earlier skill tree. For example, MAX skills might not be available until you are highly trained in Heavy Assault.
The implication is that whereas anyone (or at least, a good proportion of players) can spawn any vehicle or armour, only those who have progressed down its skill tree will be able to use it effectively.
Each class has its own skill tree to allow complete customisation within the game's several classes.