Hi Noodler
The Dominant is the 5th scale degree of major and minor scales. I didn't know the Nashville system used those terms too - I've learned something.
Anyway the 7 notes (scale degrees) are named TONIC, SUPERTONIC, MEDIANT, SUBDOMINANT, DOMINANT, SUB MEDIANT and LEADING TONE.
As you know, you can build chords on every scale note. The chords take the same name as the scale degree that they're built on so the one built on the DOMINANT scale degree (the V chord) is called the DOMINANT CHORD - if it's a 7th chord, then it's called THE DOMINANT 7th chord.
Originally, the term DOMINANT or DOMINANT 7th referred only to that chord IF it was the V or V7 chord - but things change and now the term is used for that type of seventh chord (1 3 5 b7) no matter what scale degree it's built on.
What makes this chord unique and worthy of the name DOMINANT is that it's the only one that contains a TRITONE that wants to resolve strongly to the TONIC or I chord. That chord alone defines the key.
But a TRITONE can actually resove two different ways and that's what allows you to substitute.
In the key of C major the tritone is B - F (in the V7 chord G7) - it resolves naturally to the tonic chord C major
In the key of Gb major the tritone is F - Cb (= B) (in the V7 chord Db7) It resolves naturally to its tonic chord Gb major
The two tritones sound the same as they have the same notes (although named differently to conform with their keys) and that's why you can substitute one for the other.