Here is a quick rundown on how it works. Each number correlates to a note in the key you are playing in. So if the session leader says that the vocalist on a particular track wants to cut it in the key of G the number 1 = G, the root of the key. 2= A, 3= B and so forth. The advantage here is during a session if a particular key is not working for the vocalist you can change the key instantly without having to transpose your chart by sight or have someone rewrite a dozen or so charts. This saves a lot of time and aggravation.
I have been on a few sessions in Nashville back in the 90s and it is amazing how quick the pickers can transpose, notate or make changes to a track. And what’s more is that in a take or two it sounds like they have been playing together for year! Only in Nashville..!!!!