MPEG-4 Part 14 container format, based on the ISO base media format which itself derives from Apple's QuickTime. Not a codec — a container carrying codecs and metadata. Composed of three parts: ftyp (file-type / brand identification), mdat (media data and streams), and metadata. Metadata is structured as 'atoms' in two broad groups: fixed file parameters and specific pointers to chunks of video/audio frames. Metadata is required before a player can start streaming. By convention produced at the end of the file in a single ffmpeg pass; streaming-optimized output requires moving it to the front (see faststart).