Hi Luckasa, in Prefab Tool, a Prefab is container of actors, but not meshes. E.g. if you setup a house in level which contains all the furnitures/doors/windows/decorations meshes, you can store all those static mesh actors as one single Prefab to be resued in any levels.
All prefab actors are saved in the level just like normal actors. One of the design goal of Prefab Tool is that there will be no runtime overhead when using Prefab.