What affects field upgrades in BF4?

Firstly, how do you gain the score exactly? I know it shows in the bar on the side of the map, but I remember a match where I was lone-wolfing and still got a pretty high score in that bar. I suppose my other squad members were together, but do I also gain for their work?

Also, does the score reset when I start a new game? I mean, I play support with the Offensive perk and it took me a long game of conquest to just unlock lvl-3 extra grenade. In the next match, that 'nade is gone. Don't I keep it forever?

It is done by working with your squads. Squad points are made when healing a squad mate, being a spawn point, capping a designated flag, etc.

If you lone wolf, your squad will take more time unlocking them. Same goes if your squad leader does not give orders. Only the squad part of the points will go toward your unlocks. So let's say you get a squad savior, you get 25 for the savior and 25 for the squad part, only 25 points will go in your perk progress.

You lose your progress toward your next level and a level when you wipe. If you are alone, you will wipe everytime you die.

From what I read, the upgrades are in individual, however, as I said before, they require squad play to unlock.

Level cap explaining it

All the upgrades

DICE explanation

The game changed how those perks are allocated from BF3.

If you follow squad leader orders and perform squad-based actions (killing people at a cap point marked by SL, healing squadmates, resupplying squadmates), you will quickly level up these squad perks. The key is to have an active SL who is marking points and giving you orders so that you can rack up the points as quickly as possible.

When you have a squad wipe, you lose one level of perks.