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Channels are used for grouping joints. They allow animations to play only on a subset of the model joints, so that multiple animations can play on different parts of the model at the same time.
For example, you can play a walking animation on the legs, an animation for swinging the arms on the upper body, and an animation for moving the eyes on the head.
Channels are defined and inspected in the Model Editor as described below. The use of channels is up to the game's script or C++ code.
An RMB click in the Channels pane opens the context menu: