Point taken regarding the format of each of the documents, they have been produced incrementally as each feed was developed. They are more consistent than they used to be, and we are still working on it, please bear with us 
The id 21-S9-sj2-1 is an example from the Complete GTFS bundle: https://opendata.transport.nsw.gov.au/dataset/timetables-complete-gtfs. The relevant documentation for this bundle is here: https://opendata.transport.nsw.gov.au/sites/default/files/TfNSW_GTFS_release_notes.pdf
The format of that ID is defined by the scheduling software and goes something like “OperatingBranch-RouteNumber-Instance-RouteVersion”. In general these values (apart from RouteNumber) aren’t that meaningful for consumers so they haven’t been defined in the technical documentation.
TfNSW_Realtime_Bus_Technical_Doc.pdf relates to the For Real-Time GTFS bundles for buses: Public Transport - Timetables - For Realtime | TfNSW Open Data Hub and Developer Portal