This is a modified document from Dromed Central. Author: Evilspirit.

• .mis files are levels (missions).
• .gam files are gamesys files - this is where things like the Object Hierarchy lives. Thief2 has a 16 .mis files and one .gam file; System Shock 2 has a different .gam file (with science fiction-type objects) and completely different .mis files.
• .cow files combine .mis and .gam data elements into one file. (p_portal.cow is created when you portalize and may be useful as a backup if you crash while doing so).

A .mis file contains a reference to the .gam file which is intended for use with the level. This .gam file will automatically be loaded when you load the .mis file. A .cow file needs no external reference to the gamesys because it contains all that information itself. Using .cow files while developing a level is slightly simpler that having to manage both .gam and .mis files, and may protect you from forgetting to save the gamesys after a change. However, .cow files are usable only by DromEd. You will need to save your convert your final .cow file to a .mis and .gam prior to distribution.