In overall, when we take notes we do it based on the relevance the idea has to us, that is to say, if it is something we'd need to review or investigate later on, if it has some sort of significance to us, if it is an idea or concept that is new to us or an idea we haven't fully grasp it, etc. And this sort of decisions depends on the subject of the lecture. For example, if it's a familiar subject, it is more likely that the notes we take aren't as many as the notes we'd take on a non-familiar subject, and we'd probably take them more nimbly.
