The Indoor Mapping Data Format is Apple's way of modeling how to map indoor spaces. An IMDF archive is one manifest.json file with many .geojson files which are tightly related to the id property in some of the GeoJSON.
Apple has created an IMDF Sandbox to validate and/or report any issues you may have with your IMDF archive.

Here is a summary of links that we used to learn development of our IMDF solution.