Yes, in this example the 21 in that list is the IRQ number for the USB.
The idea is that you should only have one IRQ assigned to your USB ports. If you have the same number being shared by other things then it means that the USB is competing with those other devices for interrupt requests. In some cases this can lead to problems, so if you are experiencing a problem and nothing else works than this can be an avenue worth investigating.
-Annikk
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