mp4 mdat box can include AUD ?? (Access unit delimiter) (nal_unit_type 9)

Question

mp4 mdat box can include AUD ?? (Access unit delimiter) (nal_unit_type 9)

The access unit delimiter may be used to indicate the type of slices present in a primary coded picture and to simplify the detection of the boundary between access units. There is no normative decoding process associated with the access unit delimiter

I learned that SEI(Supplemental enhancement information) can be included in mdat box
ref :
In Video Does Sample Means mdat data?

I wonder that mp4 mdat box can include AUD ?? or do i must filter out AUD when i make mdat box

if only SEI can be mdat and AUD can not be mdat Could you tell me that reason of it?

Answer 1

AUD should not be not be in 'mdat' since each AU is prefix by its size. SEI can change through out the stream if it contains realtime metadata - for example timecode. So SEI needs to stay in 'mdat' since it can't be factored out or has a replacement.