Saves, Resistance, Incantations, and Kernels
A Casting% is an amount less than which a caster must roll to succeed in casting a spell. Incantations are more complicated and yet, due to the way in which they expire upon being cast, more likely to succeed apart from conditions adverse to concentration (e.g. the caster wearing heavy armor). A Kernel is a bit of magic drawn from the nature of a spell, so that it remains an option for the character.
Against incantations, a creature is usually entitled to a save. A Save is a % chance to avoid the full impact of the magic. Generally, saves either result in avoiding all or some of the impact of the incantation, but the change varies from spell to spell.
When a creature has resistance, it means that incanted spells are more difficult to cast on it. However, one creature’s resistance does not nullify magic for other creatures. Thus, if a creature has resistance, the casting% is treated as that much lower versus it (alone). The same spell can succeed against some creatures and fail against others. Having resistance does not deprive a creature of its privilege of getting a save.
Kernels are not supposed to allow for save, but they are less reliable. Kernel casting is affected by resistance too.
The goal is to have a system where really only one roll is needed (although some math may be required), prior to determining the effects of the action. However, if the fuller ‘incantation’ is cast, it seems a significant enough event that anyone affected is usually granted a save.