There are a lot of openings that one usually needs to concentrate on just a few to learn if one is going to have a life to speak of.
Tactics are useful all during the whole game. Positional chess is applied also throughout each game. Pawn Structures are a must learn.
Learning about chess theory involves opening, middle, and endgames. Each of these three could be broken down into a variety subjects. How would you identify what subject(s) to include for any/all parts of chess theory?