The mapping from glyph indices to coverage indices is defined in the `Coverage' tables. Later on, in all GPOS and GSUB tables, everything refers to those coverage indices.
However, for a user reading the XML output of ttx, it's very hard to trace this mapping. For example, if you have an entry like this
you have to count the lines in the associated coverage table to find out the corresponding glyph index. I thus suggest to add comments so that the above looks like this:
<Class2Record index="44"> <!-- Agrave >