Difference between Scenarios and Presets?

I understand that Maps are custom-made with no pre-selected areas, but Scenarios and Presets seem to be the same thing. What am I missing?

Presets have no player sides and scenarios do. Presets allow you to block out a certain section of the map and make it so that players can only choose in the region that's not disabled, while scenarios only allow you to choose territory that's attached to a "scenario side" or player.