Assigns a local value to one of 5 data slots in the client that can be used to keep track of items that belong to this client. For example if you create a sprite to represent the client in your game you could assign the sprite ID to one of these data slots so that when you go through the client list you can detect which sprite you used for each client. This also allows you to clean up the sprite if the client is found to have disconnected. This value is entirely local and does not get transferred across the network.
SetNetworkClientUserData( iNetID, client, index, value )