There are two competing schools of thought. "Primary/Supporting" and "Control/Performance." The mistake some make (not you) is thinking they are scan techniques. They are not. They are
instrument interpretation techniques. P/S is the FAA's; C/P is from the military. For some years now, the FAA recognizes both. It didn't always, apparently figuring the armed services didn't know much about flying
.
Primary/Supporting asks, "which instrument gives you the most direct information about your condition?" That's "primary." Other instruments providing the same information are "supporting." For the easiest example, in straight and level flight, the primary instrument for pitch is the altimeter. In P/S, the Attitude Indicator is primary only while you are changing attitude since it is, after all, the most direct indicator of, ummm, changing attitude.
"Control/Performance" asks a slightly different first question. "How do you control what the airplane does?" It treats the Attitude Indicator as the "control instrument" for (surprise!) exactly the same reason P/S treats it as primary for changes. Instead of dividing the others into "primary" and "supporting" roles, it treats them collectively as telling us whether the "control" changes we made have produced the desired "performance."
Arguments over the two tend to sound like the same nonsense spouted in pitch/power arguments. They are both really nothing more than two ways of looking at the same thing and approaching the same goal - having us cross-check our instruments to know what they are telling us and resolve discrepancies between them.