The Material Editor could be called the Material and Map Editor, because you can use it to design both materials and maps, and any combination of the two. In addition, you can create material or map hierarchies.

Material/map hierarchy shown in the Material/Map Navigator

A material hierarchy is a material that consists of other materials (or maps). Similarly, a map hierarchy is a map consisting of other maps. Materials that consist of other materials are called compound materials. Maps consisting of maps are compound maps.