In the Chords pdf in Stepping Stones, it states:
"The I, III and VI are tonic and can be substituted for each other
The II and IV are sub dominant and can be substituted for each other
The V7 and VII-7b5 are dominant and can be substituded for each other
* This is because they share three tones in common with each other."
My beef is with the three tones in common explanation. All diatonic chord shares 3 notes with the 7th chords a 3rd above and below it. The IV and VI share 3 notes, as do the III and V, and the VII and II.
How about this rationale?
If a diatonic 7th chord does not contain the IV note, it is tonic.
Else, if it contains the I note, it is sub-dominant.
Else, it is dominant.
Grouping the diatonic 7th chords into tonic, sub dominant, and dominant is an established theory practice that has been in place for decades if not more. It was first introduced to me in the late 70’s at Berklee. If you don’t like it, ignore it.