Obviously enough "in" is the introduction, A is the main verse, B is the chorus, and part C is where Garcia normally takes a solo against that descending line A, G#, G, F# which corresponds to the chords A-, A-^7, A-7, D7 etc. I've not scored the transition to A and D where they sing "Across the rio grande-o" since it doesn't fit in the iReal edit window. I wish I knew how to make part C go on it's own line without inserting lots of spacees, let me know if you know how please.
Hope you enjoy, Rob