Understanding of "mask"

[nangergong]

Hi, all,

  I don't understand the meaning of "mask",
  for example, in the following statements:
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  ipfw pipe 1 config delay 80ms mask all
  ipfw add 600 pipe 1 ip from 9.161.150.55 to 9.161.148.72
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What is the usage of  "mask all"?
I think in total there is only 1 queue, so "mask all" here doesn't make any
sense?

Thank you!