At the moment, there are four zone difficulties/map locations to choose from for Save The World, the PvE aspect of Fortnite. Each location has an Outpost that players can expand, with each expansion unlocking a new group of missions. Completing all quests for an outpost unlocks the next location.
Missions themselves may be viewed on the map interface, ranging in color from grey to red. Grey-shaded missions offer no Commander XP, but players can still acquire the rewards. Red-shaded missions are the hardest.