Match is the object that holds scope-local state; i.e. state that gets allocated anew whenever you enter a new rule or parenthesized group. This is approximately the Match object that userland folks will see, but it has some other clutter because of unclear design.
$.from - The place where this subrule/group started. $.to - The place where this subrule/group ended. C<Medium> represents positions I<between> elements, so if you're thinking of these as numbers, this will be one past the last character matched. $.capture_num - Numeric captures: $/, $/, ... $.capture_name - Named captures: $/<foo>, $/<bar>, ... $.multidex - The "quantifier index". This is an array that is an index into the multidimensional arrays of the match object, telling captures where in the deep structure to store what they've captured.