Overcome: You can use Arcanum to overcome any obstacle that requires applying your character’s magical to achieve a goal. For example, you might roll Arcanum to decipher a spell formula, under the presumption that your character might have experience with similar spells at some point.
Create an Advantage: Arcanum provides a lot of very flexible opportunities to create advantages, provided you can research the subject in question. More often than not, you’ll be using Arcanum to get a story detail, some obscure bit of information about a cult ritual, magic writing or magical formula. Likewise, you can use Arcanum to create advantages to protect from the effects of Spells with ward or charms for example.
Attack: Arcanum isn’t used in conflicts.
Defend: Arcanum isn’t used to defend.