IIRC, since it is impossible to scale enemy difficulty to fit multiple players who are using different difficulty settings (e.g. Alan is playing on "Normal", Bob on "Advanced", and Cathy on "Elite"), a multiplayer mission will default to the difficulty level of the player that the server selects as the initial mission leader (i.e. the first player loaded into the mission map, unless the players are teamed ahead of time).
Thus, if whoever was "mission leader" was playing at Advanced level, then all players will be playing at Advanced level, overriding their personal difficulty settings.
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Thus, if whoever was "mission leader" was playing at Advanced level, then all players will be playing at Advanced level, overriding their personal difficulty settings.