Hi Steve,
I have just started to have a look on the internals of the garmin fileformat. The wiki (http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/Format) tells me that there are the formats TRE NOD MDR SRT GMP DEM
The documentation of the garmin-img sourceforge project also mentions the formats not yet documented in the wiki: LBL RGN NET
I think LBL is the file in which the labels are stored in a non searchable format. And MDR contains the same information like LBL but it is searchable. Am I right?
I don't know the internal format of the subfiles. But here is my overall understanding of the files. As far as I know, LBL contains labels for each map object. Its like a list of strings. Each map object points with an index to an entry of the LBL file. The MDR works the oposite direction. It contains chopped and sorted fragments of adress data, optimized for searching and input auto completion with least effort. The result is an index into the map object lists (or a coordinate?). So yes, it contains more or less the same data, but the using is different.