Overview: MP4 packaging
MP4 packaging choices affect media segmentation, startup access and the conventions used for audio-only files.
A reader may need container information before it can interpret the media bytes. Fast-start placement and fragmentation address that relationship in different ways, while M4A and M4B names describe common audio-file conventions. The linked topics distinguish physical organization from the codec and content-type assumptions often attached to an MP4-family filename.
Fragmented MP4
Fragmented MP4 distributes timing and sample descriptions across successive media fragments after an initial track description.MP4 fast start
MP4 fast start places the movie description before the bulk media data so a sequential reader can initialize playback earlier.M4A and M4B file conventions
M4A and M4B are audio-oriented filename conventions for MP4-family files, with M4B commonly indicating audiobook content.