This file is similar to the TDB file in that it is a list of maps along with other information describing the set. It is inserted into the gmapsupp.img file.
A map will work without this file, but it is essential when you have maps from different families loaded together as it allows you to see the names of the map families and turn them on and off separately.
It isn't like the other files in the app package, in that it doesn't begin with the common header. It should perhaps live with the TDB format or in a similar top level place. It is here because it is included inside the gmapsupp.img file and doesn't normally have a separate existance.
The names and lengths of the fields were obtained from the program gpsexplorer by the author identified as 'garminmaploader@yahoo.com' and released under GPL2+.