Top Layer is also called the component Layer, which is mainly used for placing components. For the double sided pcb and multilayer pcb, it can be used to arrange wires or cover copper.
Bottom Layer, also known as the welding layer, is mainly used for wiring and welding. For the double sided and multilayer pcb, it can be used to place components.
Mid-Layers can be up to 30 Layers, and are used to lay signal lines in Multilayer pcb, excluding power and ground lines.