Material Editor > Maps rollout > Drag one map button to another.
The Copy (Instance) Map dialog is displayed when you copy a map by dragging and dropping a map button. It gives you the choice of copying the map, making the newly assigned map an instance of the one you dragged, or swapping maps.
You can drag map buttons in the Maps rollout, in the Basic Parameters rollout, or from one rollout to the other.
Tip: Usually when you use the same map for multiple parameters, such as both self-illumination and opacity, it is easier to work with an instance rather than a copy.
Instance—Makes an instance of the map you dragged. The newly assigned map is not independent. Adjusting the parameters of one map or the other changes both of them.
Copy—Copies the map you dragged. The newly assigned map is a copy whose parameters you can adjust independently.
Swap—Swaps the maps. This option isn't displayed when you drag from one rollout to another.