From Pub 17 "
Offset against debts. If you are due a refund
but have not paid certain amounts you owe, all
or part of your refund may be used to pay all or
part of the past-due amount. ... You will be
notified if the refund you claimed has been offset
against your debts."
No clue given as to what the rules might be. It probably means if you have more than one debt it gets divided somehow.
