This question came over via email and I thought it was good enough that others could benefit from it.
I was researching musical theory, and came upon the Hook Theory site.
In order to see what it was, I entered a song that I know, and that I am analysing.
The Hook Theory system displayed part of the song, Sultans of Swing (Dire Straits), with an apparent “error” in the chord labeling.
The song is in the key of Dm:
i ii III IV v VI VII
D E F G A Bb C
F G A B C D E
A B C D E F G
The Hook system showed the correct chord, A major, but incorrectly labelled it as the V chord.
The fifth scale tone chord of Dm is A minor (A C E), not A major (A C# E).
It makes no mention of chord substitution being used.
I realise that the " V " might refer to making the Am (v) into an A (V),
but, it is only a " V " in the key of D major:
D E F# G A B C#
The melodic minor scale uses a raised seventh degree (C#),
and chord substitutions work best when supporting the melody, which is why, I suppose, the A major chord is used.
I’m wondering exactly what the Hook Theory system show’s and teaches, based on that example, as it would be very misleading and confusing to someone who is trying to learn music theory, doesn’t know what that material yet.