It’s been my informal experience that if a spec is available for cheap from https://www.evs.ee/en/ https://www.evs.ee/en/ then it’s often somewhat hard to find,

It’s been my informal experience that if a spec is available for cheap from https://www.evs.ee/en/ https://www.evs.ee/en/ then it’s often somewhat hard to find, but in this case 14496-15 is only listed in it’s more expensive variant and there’s no cheap one available. When that happens, I can often find a copy of the spec by hunting down zip files of drafts attached to meeting records online or I can find a draft copy of the spec after a simple registration form. I can’t speak to this one for the 2019 latest version or newer, but I did find out what might be some earlier versions at the bottom of https://mpeg.chiariglione.org/standards/mpeg-4/carriage-nal-unit-structured-video-iso-base-media-file-format https://mpeg.chiariglione.org/standards/mpeg-4/carriage-nal-... for example. Or for newer work-in-progress: https://www.mpegstandards.org/standards/MPEG-4/15/ https://www.mpegstandards.org/standards/MPEG-4/15/ No promises that anything found this way will be used/useful yet.