Map editor for Might & Magic: Heroes VI.

I understand that the mechanism there is different. The first player automatically becomes a human, and all the others are controlled by the computer.

It's not a certainty, because in "Heroes of Might and Magic V," the campaign was made up of five separate maps. I don't think it's any different here. Most likely, there's some other feature associated with "player only," like a brush for battles. Perhaps this is mentioned in the manual.