[Fireball]

Do the clients also need the mod installed or only the server?

Each client needs the mod installed. If one of the clients did not have the mod installed, that person would not have any external views.

Technically the server doesn't have to have the mod installed. As long as the paramaters are in the [Mods] section of the .mis file, they will get passed to all players that have the mod, regardless of whether or not the server has the mod installed.