When someone modifies various aspects of the game such as units, technologies and graphics etc, it is call a mod. Recently support for mods has been added to Voobly which means once the host has selected the mod and everyone has downloaded it you will be able to play it. For now you can't launch if you do not have the mod the host as selected, but we hope to add support so you are prompted to download the mod when you enter the room.

You will now be prompted to download the selected mod when you enter a game if you do not already have it and it is an official mod.