channels.
You may have noticed that nodes visually indicate the channels which they are processing (that is, treating in some way) and passing (that is, conveying without any treatment). This is done via a system of colored rectangles, which allows you to trace the flow of channels throughout a script.
Look closer, for example, at the Over node. The wide rectangles indicate channels which Nuke processes (in this case, the red, green, blue, and alpha channels). The narrow rectangles indicate channels that Nuke passes onto the next node without processing (in this case, the mattes.soft and mattes.hard).
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| Visual confirmation of extra channels. |