This literally is geojson (RFC 7946), which they do say (but don't really spell out). geojson basically give you a way to describe a geometry, state its type (e.g. 'lake'), an associate some arbitrary properties with that region.

IMDF is Apple enumerating some feature types and stating what properties are required for them. In some fields this is called an 'application profile', a particular use of a format for a specific field of use.