Sometimes the desired texture mapping is difficult to define just in terms of the rectangular texture areas. The standard texture areas are always rectangular and are restricted by their orientation on the surface. Furthermore, spreading a single texture simultaneously across multiple surfaces is not possible using standard texture areas.
In such cases, an alternative method is provided. This requires you to first define the texture on a separate bounding surface. The bounding surface is not part of the model and so will never be visualized. The texture will be visualized by being projected onto the desired surfaces.