I can't answer your second question, but the levels here correspond to years of study at the music school where Mr. Delcamp teaches. D01 covers the first year of study, D02 the second, and so on. The scores are also presented in order of difficulty within each of those levels, to help you to progress smoothly through them. They correspond roughly, though not exactly, to grading systems used by various music institutions as described in this
reply, among others.
Classical Guitar Forum.
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