Related description from Use Magic Device:
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.
Use a Wand: Normally, to use a wand, you must have the wand's spell on your class spell list. This use of the skill allows you to use a wand as if you had a particular spell on your class spell list. This use of the skill also applies to other spell trigger magic items, such as staffs.
Related description from Staffs:
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.
Then when a rogue with high-enough UMD skill tries to use a staff, what should be the proper way to determine the DC of that spell? As a DM, I don't think I should determine which ability should be applied to each of those staffs before generating them.
The ways I can think of:
- Use either INT, WIS, or CHA of that rogue, whichever is higher.
- Or, use INT or CHA for arcane spells, and WIS for divine spells.
And if the abilities are not meeting the requirements:
- If none of those abilities is at least 10 + spell level (minimum required attribute), use the minimum required attribute instead.
- Just use the highest abilities to decide the DC, even if it is not meeting the minimum requirement. (So a staff can become a worse option than a wand)
- Let the rogue make another UMD check to emulate an ability score, and use that score to determine the DC (if that score is higher than what they already have). The rogue can do this even if they have already reached the minimum requirement to have the opportunity to approach a higher DC. (Let's say somebody has INT 14 but can use UMD to emulate INT 18, then use INT 18)
- Just use the minimal DC regardless. When it comes to UMD, treat the staff as a wand.
I wonder if there's some kind of official Q&A to sort this mess out. If not, please let me know your opinion on how to solve this.