Staff ability score and multiclass casters

[Mathias Gehl]

This is not something that will happen often, but let's say a character is a Wiz 12/Clr 8, but has a higher WIS bonus than INT bonus.

For the purpose of spells on a staff, can they use the Wizard CL and the Cleric ability score bonus? Or do they use the ability score bonus that's linked to whatever class is providing the level they're applying to the staff? Or can they choose which class' stats (level + ability score) apply to the staff for this use?

What if their STR bonus is higher than INT or WIS, can they use that?

All it says in the staff description is "Staves use the wielder's ability score..." - I can't find anything that says which ability score.

[Tom Baumbach]

The ability score used is the relevant ability score of the character's caster level-giving class. A character chooses which class (thus caster level, thus ability score) it is using when activating the staff.

A character cannot use an ability score that is unrelated to its caster level.

In answer to your specific situation: the character could activate the staff as a wizard (using Int) or as a cleric (using Wis), but not a mix of the two, nor could he substitute any other ability score.