The tritone substitution has been discussed in several questions here, perhaps most notably What is tritone substitution? and Why is bII 7(b5) considered a dominant? (I believe my question is different, but I'd understand and respect the urge to mark this as a duplicate.)
My understanding has always been that "tritone substitution" means that we use a dominant seventh chord that also has the same tritone found in the regular V7 chord. In C,
G–B–D–F is our normal V7 chord. But we can instead play
D♭–F–A♭–C♭, because the
B–F tritone of the G7 becomes the
F–C♭ tritone of the D♭7.
But the answers in the linked questions all emphasize that the chords have a root a tritone away from each other, and that these "usually" involve dominant seventh chords; it's only mentioned later (although not always) that they also happen to share the same tritone.
I ask because—and here's why I think my question is not a duplicate—if "tritone substitution" means the first definition above, then it must be used with dominant seventh functioning chords. But if the definition is the second definition, then we can use any chord qualities. Needless to say, a proper understanding of this definition could prevent some major confusion regarding (and incorrect usage of) tritone substitutions, and I don't believe the current questions clarify this distinction.
Another way of asking this question would be: does the term "tritone substitution" refer to the interval shared between two chords, or the root distance between them?