If an object will be seen up close and requires lots of detail at some point in the game, but it will be far enough away at other times to needlessly tax the engine, one often uses an LOD stand-in. At a particular distance, the high poly object is hidden, and the low poly stunt double is shown in its place, to improve frame rate. With some engines, this may be handled by the engine itself creating the lower poly version and swapping out at a pre-determined distance. In other engines, or with important meshes, the artists will provide the LOD version or versions.