Each spell has an associated Magic Attribute, which determines the attribute bonus used for Spell Bonuses, Spell Accuracy, and Spell DC by association.

The Magic Attribute is normally Arcane, Occult, or Divine.

As such, Magic Attribute Bonuses are just the bonus of the relevant Magic Attribute for the spell.

So, if Arcane Lance is a spell attack, and it lists it’s Magic Attribute as Arcane, then you use the bonus for your Arcane attribute as the accuracy bonus when you make the attack roll for that spell.