This is a DASH specific box. See [1] for its definition.
It will probably be useful in a near future, and I will use it at a
workaround to get fragment duration when tfhd and trun boxes don't give
(default) sample duration(s).

[1] http://www.3gpp.org/ftp/Inbox/LSs_from_external_bodies/ISO_IEC_JTC1_SG29_WG11/29n12310.zipSigned-off-by: Jean-Baptiste Kempf <jb@videolan.org>