Power modification stems from the ability to grant "names" to other people's abilities (or your own abilities), changing their properties in the process. For example, it has been used to give states of being like spatial intangibility a hard time limit, or turning attacks like punching a hole in someone's chest into something non-lethal. This doesn't remove the non-altered ability though, so a character affected in this way could still use their unmodified techniques so long as they know they can do so.
Resistance modification is a trait that youkai have, where they can simply rewrite what does and doesn't affect them. The clearest example of this is Suika using it to ignore weakness to beans (which usually burn her flesh and can exorcise her) and proceeding to eat an entire plate without issue. So for Remilia, she could probably choose to stop being weak to the sun, as an example. Although, this ability is extremely out of character because youkai rely on on people's beliefs in the rituals used to exorcise them to continue existing (eg; People believing vampires are weak to the sun, garlic, silver, and so on is part of what keeps the legend of vampires and the actions used to keep them at bay alive, and thus keeps Remilia alive as well).
Not to say any of this gives Remilia a win, though.