Every class that casts spells, manifests powers, or uses some other magical system has one specific ability score called their Key Ability on which their magical abilities are based. The Key Ability Modifier typically determines the highest level spell, power, or ability that can be used as well as the save DC of the ability.
Different system may use a specific name for the key ability, such as veilweaving modifier or initiation modifier; however, these are all effectively the same thing.