Something along the lines of the mod will read your iwd and then go read the mods iwd . From that the game figures out what to load and how to play the map and mod. The arena file is part of what it goes to read. So if your arena file has a list of maps with game types IE map rotaions, different ways of naming your map , different maps for different game types. If its a balnk arena file the game thinks your the only map . Well something like that. This way the mod does not go and read all the other maps and loads all the other textures and fx's and so on. It's a bandaid fix at best and might not work. The other way is to make small map packs so what the mod reads is smaller. Its a mess.