Difficulty level, roughly compared to ABRSM exam grades. 0 is beginner, 9 is advanced (beyond grade 8).
taken from the Composer's Note:
'The book is designed to break through the barrier of "I can't improvise" and get the player going almost in spite of themselves! Reference is made to various scale types and modes, but the book is designed to draw on instinctive reactions, rather than technical know-how. Some explanatory notes on chords and functional harmony is given, but reference to other texts is also encouraged.' Christopher Norton