A complete chess program must incorporate 3 different skill sets.
These are the basic elements that are to be understood by every single player. It does not matter what their playing strength is, this they need to know.
This would be a targeted exercise, customized to the needs of the specific player. They are designed to cut their flaws.
This one is the most difficult one to work on because of the plenty of misleading information on the subject. It deals with thought processes and decision-making in practice. It also deals with incorporating this into one operating procedure during playing.