PCs have a number of brain slots equal to their Intelligence defence. Brain slots are used for proficiencies, be it use of armor, brewing potions or sneaking around. Whenever you are trained by an NPC or fellow PC that is higher level than you, you gain one proficiency rank, which fills one slot. Whenever you make checks pertaining to that proficiency, add +1 for each rank. Generally, it takes 1 week * rank number to become trained in a specific rank. If your brain slots are filled up, you can elect to forget a rank in a proficiency to learn another. Doing so doubles the time required to learn a new rank.
Alchemy - craft a single item of ink, soap, oil or tar by passing an Intelligence saving throw. For each new rank, you can make 2 more items with this proficiency. Crafting alchemy items usually takes 4 - 8 hours, depending upon your preparation and work space.
Armor - choose an armor type to become proficient with. You can force enemies to re-roll attacks a number of times equal to your proficiency. For each 8 hours of rest, you regain one spent re-roll.
Elemental Magic - you've become proficient in the use of elemental magics. You can add your proficiency ranks to spell damage and attack rolls.
Toughness - you're just really tough as nails. Add your ranks to hit dice rolls and maximum hit points.
Credit: Sigve Solvaag