Sublevels (Map Entities):
A map entity is simply a small compiled level, stored in an external WMB file. Map entities can be used for level parts which move as a whole, like doors, platforms, or vehicles. Because they are nothing else than compiled maps, you can create them using by WED. You can, by example, convert each prefab into such an entity by loading it as a Map, performing a BUILD as Map Entity, and copying the resulting WMB file into the current level directory. You'll find many predefined doors, furniture, vehicles and thse like in the prefabs subdirectories.