Musical Relationships are Interval Distances
The sound of a note of a melody, why does it sound the way it does?
Firstly, there is the idea of technique, the "way" the note is played. Loud, muted, soft, bright, dull, with a sense of vibrato or without. Long held note, or staccato.
This is like a "stylism", and is often a reference to a musical style. It's also a component of the original character of a guitarist, their sound.
And then we have the idea of a musical distance. And this is where it gets complex. If only it were not so. And yet it is complex. So let's start with a simple idea. The musical distance of a semitone is a single fret.
The musical distance of a tone, is two frets.
The key of a song might be E, and so the tonic is the note E.
Seven chords form the Harmonic Boundaries of a modal key signature, and again we have the idea of a tonic note, only this time, it's the tonic note of each chord. Say for example E Phrygian Mode.
The seven "Modal Chords" of E Phrygian Mode:
There we have seven tonic notes.
If a modal guitar chord progression consisted entirely of a single chord, E min7, played in different ways, then to measure the "musical distance" of any melody note for the song, would only require a comparison of the melody note to the tonic note of E min7.
That might or might not be slightly unsatisfying if the song just had a single chord. Okay so let's put a few more chords together. Let's say three different chords. Now there are three tonic notes. So there is the ... shall I say, necessity? to measure the "musical distance" of any melody note for the song to tonic note of the key signature (E Phrygian), and to the tonic note of the chord that it sounds over in the progression (the tonic note of the chord).