Think of your car (if you have one). It is a car as long as you can drive it. But if it breaks down, rusts or is destroyed in a crash? Or pressed into a lump of metal in a scrap press or molten in a furnace. These are normal processes for the underlying system, but the car is no longer a car. Actually, the car you know is just in your imagination. You pretend as if that object is your car and that works as long as it is working as a car. The underlying system (i.e. the “real” physical system) emulates your car, i.e. it behaves according to the car-theory (the construction made by the engineers). But its parts are not described completely by that theory. For example, they develop cracks or rust. So, for some time, they behave as car parts and as long as they do, you can drive.