Before dice are rolled for spellcasting, the caster must consider if her subject can Withstand her magic, as it could affect the dice pool and outcome of the casting. The subject of a spell can always choose not to Withstand a spell, but she must consciously choose to do so as a Reflexive Action.
If a spell must overcome some aspect of its subject to take full effect, it will list a Withstand rating in its description (usually a Resistance Attribute, but spells themselves Withstand dispellation using the Arcanum rating of the caster’s highest Arcanum used in the spell).
Withstood spells reduce their Potency by levels equal to the Withstand rating. If this leaves the spell with no levels in Potency, it still counts as an active spell against Spell Control but has no further effect. Spells with multiple subjects apply their Potency against the Withstand rating of each individual subject, so may take effect against some of them. Each spell in a Combined Spell is Withstood separately.
If a spell has multiple Withstand ratings (for example, a Withstood spell cast with the Sympathetic Range Attainment) it uses the highest rating, +1 for every additional rating.
Characters may only spend Willpower to increase a Resistance Attribute used for Withstanding if they are aware of a spell being cast upon them.