A unit is interpreted in two different ways. Casual players will probably understand a unit to be the moving objects on the map like infantry, boats and siege weapons. However, scenario designers, modders etc. will be more likely to think of a unit as any object on the map. This includes gaia/nature objects such as trees, mines etc and buildings, as well as mobile units. Each unit belongs to a specific unit class and usually has a unique appearance and specific role in playing. Some units do not have their own unique graphics and/or sounds (e.g. some heroes) while others will have multiple appearances (e.g. buildings after advancing an Age). Some units (especially beta units have neither graphics nor sounds associated with them.