I personally think that calling NNS the Nashville Number System implies more simplicity than it encompasses. I really try to avoid semantic pedantry, but I think of it as the Nashville Notation System, since it includes a way of notating the form of a song, the overall harmonic structure, ad hoc breaks and deviations from the starting patterns, methods for notating signature riffs, methods for indicating specific rhythms, etc. I have no interest in trying to rename it, I'm just saying that the naming can affect how we think about things.So the nashville system is more or less the same as the standard roman numerals system except that it uses arabic numerals?
IOW, when you look at a NNS chart before you've learned the system, you recognize a single part of it, which is the Arabic numerals that you learned in kindergarten. But just because that's all you see and comprehend, that doesn't mean that is all that is being communicated.
I cannot think of a way for a person to understand all the things that NNS encompasses without learning NNS. I know that sounds silly to state it that way, but some posters in this thread seem to be saying something along the lines of, "I cannot be bothered to learn NNS until I know what it is". My only response is that if you want to know what it is, it's not that hard to learn.
This assumes you have a moderate understanding of harmonic theory, of course.