In the Material/Map Browser, a "base material" or a "base map" is a template for further design.
The Materials and Maps groups in the Material/Map browser contain base materials and maps. These are raw types you use to create more sophisticated materials or maps. These are essentially templates: They might have default values that will render, but in general you need to change their settings and map settings to get a good effect.
In the Slate Material Editor interface, when you drag a base material or map from the Browser into the active View, the Slate Material Editor creates a copy of the base material or map. When you drag an edited material or map (for example, from a library) into the active View, the Slate Material Editor asks whether you want to work on an instance of the material/map, or a copy.
