indexRange contains Segment Index Box (sidx). It is basically metadata fragment that contains information about the fragments the player should request when downloading the file. This is a quote from the spec:
"indexRange specifies the byte range that contains the Segment Index in all Media Segments of the Representation. The byte range shall be expressed and formatted as a byte-range-spec as defined in RFC 2616, Clause 14.35.1. It is restricted to a single expression identifying a contiguous range of bytes."
sidx box is described in HERE and a parser can be found HERE and HERE(from dash.js project)
Initialization - According to spec:
"specifies the URL including a possible byte range for the Initialization Segment."
Hope it helped!