When you paint on a single surface, Maya does not create a switch node.

When you paint on multiple surfaces with the same material assigned, Maya creates a switch node for each painted attribute. Maya creates:

• a tripleShadingSwitch for each full color (RGB) attribute, such as Color, Transparency, and Incandescence.
• a singleShadingSwitch node for each single channel attribute, such as Bump Mapping, Diffuse, and Translucence. If you paint color on one of these attributes, it automatically displays as grayscale.

Switch nodes allow multiple surfaces mapped with the same texture to be assigned to a single material. For details about using switch nodes, see the Rendering guide.