msk files are used to literally mask the area to prevent player from walking over it.
Whenever there is water or any other impassable area on the world map. So, what's stopping the player from walking over the water area? So there are 20 FRM files each is 350x300 pixels. The world map is a group of FRM files in order: WRLDMP00.FRM - WRLDMP19.FRM.
