I was wondering what the game client.exe files are for MM8BDM or Doom 2 for that matter as well? I am clueless on this area of knowledge.
So to break it down:
default.ini - Default settings for the game that is used if you are new to 8bdm
megagame.wad - iwad needed in order to run the modification. This takes the place of Doom.wad, doom2.wad, hexen.wad etc.
MM8BDM-v4a.pk3 - Contains the bulk of all the games assets. This is where all the critical files are.
fmodex.dll - Used by Zandronum. Audio related I believe? (don't hold me to that)
MM8BDM.exe - Launcher for 8BDM. This merely runs Zandronum.exe with the proper switches for the various modes. Speaking of:
Zandronum.exe - The actual engine the game uses. Without assets such as Doom2.wad or Megagame.wad + MM8BDM-v4a.pk3 this would do not much on it's own
Readme.txt - Self explanatory. Read it. Love it. Worship it.
skulltag_actors.pk3 - additional files needed for Zandronum.
zandronum.pk3 - additional files needed for Zandronum. Between this and the above, it adds additional items not present in core games.
title.png - The title picture in the MM8BDM.exe launcher
skins folder - used to store items that are autoloaded when the game starts. Such as skins.