Darkspore is an action RPG game, as we all know, and it has its own set of difficulties and campaign levels. Each difficulty is like a play. The stages are like acts (1-1--4,2-1--4 and so on) and the threats are the scenes, basically a simple play layout.
However, on each difficulty the order of scenes changes. In Onslaught and Invasion, they both have the same planet two times in a row. On the other hand, Apocalypse has planets in a seemingly random order.