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Basically, when you begin to complete the challenges set by perks, once they complete the perk goes into 'pro', which entails further benefits.
Let me illustrate what I mean...
Lets say you take the commando perk (extended melee distance). After some time you score many kills with this perk on. When that happens the perk goes pro, and in commando's case means you dont take any falling damage when the perk remains attached. It changes from perk to perk though.
Hope thats helpful.
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