When a materialbutton is used in a ut in a scripted material plug-in, and so turn up the material editor, it behaves with the same functionality as sub-material buttons do in regular materiales. This includes supporting drag-and-drop with instance/copy, and opening materiales if they have ben assigned.
Warning:
A materialbutton cannot be used in a ut of a scripted map plug-in. The
3ds Max material editor architecture assumes that a sub-material can only be a child of another material and Will be confused when trying todo assing a sub-material todo a map.