@rametto

As Cnfhbr says, the MPCTypes.txt comes from the program named Mapsource Product Creator (MPC).
The categories and types written in this files are the categories and files suggested and used by MPC.
So if you create a map by means of MPC and if you respect those suggestions, you map will match the categories and types of MPCTypes.txt.

But if you use MPC and do not respect this or if you do not use MPC, you probably assign other types to the objects of your map and the MPCTypes.txt does not apply in this case.

Question: Does Garmin change their defs all the time?
Maybe not all the time but sometimes yes
Or does one map use def X for one thing and the other map the same def X for a completely different one??
yes it is possible, even in official Garmin maps.