Selectable perk set is a dynamic set of unique bonuses that player can select or switch anytime. They can be represented as Aura-like(party-affecting/area-wide) or Mantra-like(personal bonuses to perk owner).

Only one perk from the set is typically allowed and they cannot be upgraded(but external modifiers may affect them from skills or items).
Each class has its own set of perks, and faction bonus might include "faction perk set".
Some items might add their own perk sets(represented as dropdown menus in item attributes where one perk might be selected(as switchable attribute essentially))
some mercenaries/minions/pets may allow to set their primary/secondary perks to diversify builds.
Skills might introduce passive perks set scaled by number of points spent in them,i.e.
X skill will add a dropdown menu where you select the perk from a list(typically upgrades to specific skills component, like radious of fireball or extra freeze duration), this allows to create personalized skills, geared to players style(component-skill system is more versatile overall, since points to spend on arbitrary components allows more exact customization/balance)