This is a post about terminology: whether a rule should say “bonus,” “modifier,” or “penalty.”
A bonus is always positive or zero. A modifier may be positive, negative, or zero. A penalty is always negative or zero.
If you want the rule using the character’s ability score to be a benefit and never a hindrance, use “bonus.”
If you want it to be a hindrance and never a benefit, use “penalty.”
Only use “modifier” if you really mean for it to sometimes be a bonus or sometimes be a penalty, depending on the relative plus or minus of the ability score.
Okay, out of context that doesn’t make much sense. Let’s have an example.
The paladin’s smite evil ability says:
Smite Evil (Sp): blah blah blah … In addition, while smite evil is in effect, the paladin gains a deflection bonus equal to her Charisma modifier (if any) to her AC against attacks made by the target of the smite.
If you’re playing a paladin, Charisma is a pretty important ability score (divine grace and your ability to cast spells use Charisma), and the character is almost certainly going to have a good Charisma, and therefore a positive Charisma modifier (barring Charisma damage or drain). So why say “modifier” when you could say “bonus”? Not only are they mathematically the same most of the time (barring Charisma damage or drain), but saying “bonus” in this case avoids the awkward situation where the paladin has significant Charisma damage or drain and actually has a negative Charisma modifier. Under those circumstances, does she get a deflection penalty to her AC? No, she’d get no benefit at all. If you changed that sentence to say “Charisma bonus” instead of “Charisma modifier,” you’d avoid that problem and would still have the same mechanical effect.
Note there are cases where you really do want it to say “modifier” rather than “bonus” or “penalty,” because you do want to allow for positive or negative results. Attack rolls use your Str mod. Saving throws use your ability mods. Your AC uses your Dex mod. And so on. For those rules, we want the opportunity for plusses or minuses to be valid options.
If you want something to always be a benefit, or at worst a +0, use “bonus.”
If you want something to always be a hindrance, or at best a +0, use “penalty.”
If you really want it to potentially be a benefit or a hindrance, use “modifier.”
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