Startup skills will only activate at the start of the battle. These skills are guaranteed to activate, but the chance of the effects being applied to each monster is determined separately. Startup skills will be cast in the order based on each monster's SPD. Any parameter changes that are applied will take effect in the next turn.
Active skills activate during the course of each turn. These skills have an activation chance with each monster casting one active skill per turn. If a monster has multiple active skills, a random one will be chosen and calculated by probability. Whichever skill casts first will count as that monster's action.
Passive skills do not have an activation chance, and cannot be disabled. Some passive skills will increase a particular parameter, while others may modify the properties of the monster's regular attacks.
Ability skills are a hybrid of an active and passive skill. These skills have an activation chance and the probability to cast it is done for every turn. Some skills are made to activate just as the turn begins, while others are made to activate when the monster with the skill takes action.