Four of these skills are classified as Main boosters. These boosters add +3 of the corresponding stat and then additional +50% (rounded down). Afterwards, additional stat offers will also have the extra +50% whenever taken.
A brute can obtain up to three of these boosters and can have any combination of them, but once the third booster is obtained, that brute won't be offered the fourth booster.
The main boosters are:
The other two boosters increases the corresponding stat by a larger amount than main boosters but come with a negative effect to level things out.
The double-edged boosters are: