Field upgrades allow you to customize your class beyond just weapon attachments. There are three universal field upgrades that any class can equip: Defensive, Offensive, and Shadow. In addition, there are two specific field upgrades for each class making it so you always have five options to pick from (shown below).
Each field upgrade has four levels of progression. The first level is always unlocked by default but to unlock the other three levels, you have to complete squad based objectives or tasks such as reviving squad mates, dispensing ammo, or repairing a vehicle that a squad mate occupies. You will continue to progress through the field upgrade levels as long as your squad sticks together and is alive. If your entire squad gets wiped out you will lose your progress on that bar . It is thought that you lose one field upgrade progression if your squad wipes, more on that as details become availble. DICE has made this a huge incentive to work together as a team and stay alive, revive your fallen teammates, and to seek shelter if you’re the last person in the squad so that you don’t lose your progress.