This box is top level. This box was added to ISO-BMFF standard for streaming. moov (MovieBox) contained the metadata for the whole movie but moof was added to allow the tracks (Audio, Video, and Text) to be broken up into fragments (That's why it is called Movie Fragment Box) and contain the metadata for each track. There MUST be one of these boxes per fragment.
https://observablehq.com/@benjamintoofer/iso-base-media-file-format