An object slot is a location in the console memory that stores an object. There are 240 object slots in total that may either be occupied or unoccupied by an object. The slots that are occupied are arranged before the slots that are unoccupied. If all the object slots are already occupied and an object tries to load, the game will crash, unless the object is an unimportant object such as a dust particle.
Effect of Cloning
Main article: Cloning
Cloning occurs when Mario holds an empty object slot. This can happen if Mario continues to hold an object slot after the object has unloaded. When a new object loads into this slot, Mario will hold a reference to this new object.