I don't think comparing title vs body is particularly useful. They're both important; it's acceptable to write partial answers that address only one important part of a question, but it's best to address the whole question.
I've seen a few common cases:
- The title and body conflict: they're different questions, and a single person is likely only to ask one of the two. In this case, get clarification before answering, probably by asking the OP. (In some cases you can figure out what they actually meant, and edit yourself.)
- The title includes a question that's not in the body, but the two make sense together. This happens when people are a bit careless; most likely, they figured that since they put something in the title, they don't need to put it in the body. A good answer will address both the title and the body. (A partial answer could address one or the other, but it'd be a partial answer - acceptable, but not as good.)
- The same question is phrased a bit differently in the title and body. This is pretty much just the easy version of the previous bullet: take the title and body together to understand what the OP is asking.
Bottom line, this is really not much different from asking "what do we do if there are two different questions in the body?" The answer is, unsurprisingly, figure out what the OP wanted to ask and then address it - all of it, preferably.
Sure, it's acceptable to write partial answers, in that they probably won't get a lot of downvotes, and shouldn't get deleted. But "acceptable" is a low bar. Being able to "get away" with a partial answer doesn't mean that the aspect of the question you didn't cover wasn't important, and people may well downvote answers they think overlooked an important part of the question.