The source of texture coordinates for mapping the material property’s image contents.
A geometry can have multiple independent sources of texture coordinates, each of which defines a unique mapping channel number. You can use these channels to map different visual properties of a material in different ways. For example, a geometry representing a picture frame might use one set of texture coordinates for mapping the materials of the frame itself, and another set of texture coordinates for placing a picture into the frame.
For information about creating geometries with multiple texture mapping channels, see SCNGeometry.