Difficulty level, roughly compared to ABRSM exam grades. 0 is beginner, 9 is advanced (beyond grade 8).
"On the whole, the cultures of guitar players and wind players are totally different. Guitar players often prefer rock while wind players often prefer jazz. There is however one other remarkable difference; one of the first things a guitar player learns is the E major chord whereas a wind player is usually practising the sounding notes Bb and F.
"So when a wind player wants to play along with a guitar; he will be confronted with uncommon notes F# and/or C#. Also the guitar player will be confronted with the notes Bb and/or F. Playing in F for the guitar player is easily done by tuning the guitar a semi tone lower.
"....What is considered easy for guitar players is experienced as quite difficult for wind players and vice versa. In this book we have tried to find a compromise with the goal to bring these two cultures together."