Class Ability
When an adventurerAdventurerEach character created by a player is an Adventurer. Each adventurer is a party member. performs this action, they spend the required amount of tenacityTenacityA resource tracked using the peg slots at the bottom right side of your adventurer mat. Accumulated tenacity remains until it is spent to trigger a class ability or reduced by another effect. to trigger a class ability. You may gain 1 light fatigueFatigueThese dice are placed in your adventurer's cooldown track when they gain fatigue. There are 2 types of fatigue (light fatigue and overfatigue). to change battle formBattle FormsBattle form icons appear on skill lines, abilities, and weapon items. During an engage action, the adventurer's battle form limits the dice that can be rolled together and sets the adventurer's range. before resolving the ability. You can only trigger each non-enduringEnduringDoes not require tenacity to be spent. Instead, it is active while your tenacity is equal to or greater than the required value. The ability is no longer in effect if your tenacity drops below this number. class ability once during each turn at its correct timing specified in its ability text.
Note that some class abilities are not actions. These class abilities have either the EnduringEnduringDoes not require tenacity to be spent. Instead, it is active while your tenacity is equal to or greater than the required value. The ability is no longer in effect if your tenacity drops below this number. keyword and are ongoing or have the InterruptInterruptTriggered according to its text's timing, either during a unit's turn or outside battle. If another ability or game effect would be resolved, the interrupt ability is resolved first. If 2 interrupt abilities would be resolved at the same time, the party decides which ability is resolved first. keyword and must be triggered at the time specified in the ability's text.
Resolving Partial Skills and Abilities
When you resolve a skill or ability, you must do as much of the skill or ability as is possible, ignoring any portion that you are unable to resolve. If the skill or ability has a cost associated with it, you must be able to fully resolve the cost in order to resolve the skill or ability. If a skill or ability is worded such that you must do one thing to do another, or do one thing then do another, the first portion of that skill or ability is considered to be the cost.
SourceDo I have to use an Enduring or Innate ability?
Even if they do not use the word "may", innate abilities (from class sheets) as well as any "Enduring" class abilities, are optional. You are not forced to use them if you do not want to.
SourceWhen can class abilities be triggered?
Inside of combat, a class ability can only be triggered on your turn as an action. Exceptions to this timing will be prefaced with Interrupt and provide the timing for the ability.
Aside from Interrupt abilities that specifically state they can, class actions cannot be used in the middle of another action, such as a move or engage.
Outside of combat, class abilities can be triggered at any time, as long as the ability makes sense to be triggered outside of combat and doesn't have timing specified. There is no limit to how many times you can trigger different or even the same class ability outside of battle - you are limited only by the amount of tenacity you have. The exception, again, is Interrupt class abilities. These can only be used once per the specified timing.
Source