(content element) is (content element value for independent) but (container element for IPF) or (container element for IF) does not indicate this frame is independent.
ISO IEC 14496-12 (MPEG-4 ISO base media file format):sample_number: Fix the muxer in order to correctly fill stss (if non fragmented MP4 file and USAC AudioPreRoll is present) or trun (if fragmented MP4 file and USAC AudioPreRoll is present) or MP4 sbgp (if AudioPreRoll is not present) with this frame number, then remux the content.
moov/trak/mdia/minf/stbl/sbgp: "Sample to Group"
moov/traf/sbgp
moof/traf/trun: "Track Fragment Run"
ISO IEC 23003-3 (USAC):usacIndependencyFlag: "Meaning of usacIndependencyFlag"