As far as I can tell (and fair warning, I may be wrong about this), Nashville “chord charts” aren't actually chords or charts per se as much as they are a way of being able to play a song in any key. If their intention was to not have to read music they made it a lot harder on themselves.
In a nutshell, whatever key a song is, is #1. Then the other numbers are the chords according to the scale in that major key. So, for instance, if you are all agreed that you want to play in D major, then you'll have this:
1 = D 2 = E 3 = F# 4 = G 5 = A 6 = B 7 = C#
Now what I can't tell you is whether or not they distinguish between major and minor. Technically, if you are in D, as I'm sure you know, things should look like this:
1 = D 2 = Em 3 = F#m 4 = G 5 = A 6 = Bm 7 = C#dim
Maybe they write “2m” or something, I don't know. If they assume it's a major scale and use the normal minors, then what do you do when you grab a chord outside of the scale. How would a Bb fit into the key of D, for instance?
I can tell you that, to make matters more interesting, “7” “9” or even “sus” may come after the number (“1sus” for instance would be Dsus in our example) and that they also use the ”/” symbol to indicate a bass note other than the one normally used. “1/5” would be a D chord with the A note in the bass. “4/1” would be a G chord with a D in the bass.
When I was in Europe I ran into something kind of like this in that someone would tell me that a song was in “Re” or “La Minor” using the “Do, re, mi…” as substitutes for C, D, E… but at least that didn't shift for each key. My guess is that a lot of Nashville folk played backup to numerous singers and had to be able to play a song in whatever key was best suited to a given singer. It's simply transposing, that's all.
Now, “Nashville tuning” is another matter altogether.