Sorry if this sounds like a basic question but I have a game with multiplayer incorporated and I wanted to put it on steam but I'm not sure if steams supports the C2 multiplayer object.
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I doubt that it does. But I have never used.
Steam doesn't support it in that it doesn't interact with any of Steam's features, but as long as multiplayer is working with your NW export then your game should work fine when released through Steam.