Using a Staff with Use Magic Device
In 3.5, there are no exclusive skills. In 3.0, a Cleric with the correct domain might have Use Magic Device. Regardless, the question would still apply to any character with Use Magic Device.Hmm, interesting question. Let's copy in the passage from the SRD to reference.
Staffs use the wielder’s ability score and relevant feats to set the DC for saves against their spells. Unlike with other sorts of magic items, the wielder can use his caster level when activating the power of a staff if it’s higher than the caster level of the staff.
This means that staffs are far more potent in the hands of a powerful spellcaster. Because they use the wielder’s ability score to set the save DC for the spell, spells from a staff are often harder to resist than ones from other magic items, which use the minimum ability score required to cast the spell. Not only are aspects of the spell dependant on caster level (range, duration, and so on) potentially higher, but spells from a staff are harder to dispel and have a better chance of overcoming a target’s spell resistance.
OK, obviously the activator will need to emulate the ability to cast the spells, and thus use the staff. But, since the activator does not have any caster levels, the spells from the staff would be at the staff caster level.
Secondly, Staves use the attribute of the activator to set the save DC. This is what applies to UltimaGabe's question. Because the spells are on the Sorceror/Wizard list, either Cha or Int can apply. It doesn't matter which one is used, because the emulated attribute will still be the same number and still provide the same bonus. ie. If you are using a staff to cast Chain Lightning (6th level spell), and you roll a UMD check that provides a 16 for the ability score, it doesn't matter if it is Int or Cha, both provide a +3 bonus to the DC. So, it doesn't matter which one the activator chooses.
The Emulate Ability Score aspect of Use Magic Device in 3.5 is as follows:
Emulate an Ability Score: To cast a spell from a scroll, you need a high score in the appropriate ability (Intelligence for wizard spells, Wisdom for divine spells, or Charisma for sorcerer or bard spells). Your effective ability score (appropriate to the class you’re emulating when you try to cast the spell from the scroll) is your Use Magic Device check result minus 15. If you already have a high enough score in the appropriate ability, you don’t need to make this check.
By the book, you use this for scrolls. Some might argue that it doesn't explicitly state that you can use the skill to emulate the ability score for anything else, so you cannot. We will ignore the arguments on semantics and assume the intent is to allow you to emulate an ability score for any magic item you want to activate. The key part of the skill is that you don't need to emulate an ability score if you already have a high enough ability.In this case, if the Cleric had a high Cha or a high Int, there might not need to be an emulate ability score check. If the cleric does need to emulate an ability score, it doesn't matter which one is chosen (Cha or Int) because the skill check is going to determine if the ability score emulation is high enough.
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