CHARM is covered by some early papers of Geraint Wiggins et al (I don't find it online any more). It is an abstract representation for musical analysis. Many elements specific to printed representation of music are ommited.
Unlike most music notation formats, CHARM defines an ID for each event. Structure is defined by one or more references to those IDs. This idea allows to represent almost everything without changing basic definitions. The price you have to pay for this flexibility is that you have to generate your own structures out of this data to get efficient algorithms. (This is not a bad thing. I do that in some cases in MusiXML)
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