A map channel associates a map with an object's mapping coordinates.
When you turn on Generate Mapping Coordinates for an object, the coordinates use map channel 1. You can assign new map channels with new mapping coordinates by applying a modifier such as UVW Map or Unwrap UVW to the object. Map channel values can range from 1 to 99.
Left: Scene uses different map channels to place different copies of the same maps in different locations.
Right: The three maps used to create the streets and the traffic markers painted on them
For NURBS surface sub-objects, you can assign a map channel without applying UVW Map. The surface sub-object has a different set of mapping coordinates for each map channel you use.
A map's map channel value identifies which of an object's mapping coordinates to use. Different map channels allow maps for the same object to use different coordinates. For example, you might use one channel for Diffuse Color mapping and a different one for Bump mapping. Map channels also let different maps use different coordinates within a compound material such as Multi/Sub-object or a compositor map.
Different map channels can have different U and V tiling values, different U and V offsets, and so on. In the UVW Map modifier, you can also set different map channels to have different mapping types (planar, cylindrical, spherical, and so on).
If you apply a map that uses a certain map channel to an object that has no mapping coordinates for that channel, the map doesn't appear on the object. When you render, a Missing Map Coordinates dialog appears to warn you of the problem. The dialog lists the map channel and the object name.
