The FD session group box is optional, although it is mandatory for files
containing more than one FD hint track. It contains a list of sessions as
well as all file groups and hint tracks that belong to each session. An FD
session sends simultaneously over all FD hint tracks (channels) that are
listed in the FD session group box for a particular FD session.
Only one session group should be processed at any time. The first listed
hint track in a session group specifies the base channel. If the server has
no preference between the session groups, the default choice should be the
first session group. The group IDs of all file groups containing the files
referenced by the hint tracks shall be included in the list of file groups.
The file group IDs can in turn be translated into file group names (using the
group ID to name box) that can be included by the server in FDTs.
author  Alexander Simm

A group ID indicates a file group that the session group complies with.
return  all group IDs for all session groups

A hint track ID specifies the track ID of the FD hint track belonging to
a particular session group. Note that one FD hint track corresponds to
one LCT channel.
return  all hint track IDs for all session groups