NFL contracts include several different ways a player is paid. The "base salary" is their guaranteed money, so if they're hurt during the season they would still receive this amount. Contracts also include "escalators" related to amount of time played and stats. Basically players receive bonuses for being on the field more and playing well. Then there is off season stuff, roster bonuses (if you're still on the team as of March 1 you get a bonus) and work out bonuses (show up to these voluntary practices and you get a bonus). These are really just ways for accountants to spread out payments and allow them to work within a fixed Cap amount. The team can only spend a certain amount per year on salary, but these various payment methods allow them to (legally) cook the books as it were.
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