Surprise Reopening (Unscheduled Reopening)
A Surprise reopening is one of the types of tap issues that is typical of the US government bond market.
In essence, if after the end of the auction for new government notes, a coupon rate, interest payment dates, and a repayment date fully coincide with the given characteristics of a government note, which was issued earlier, the new note will be reopened or it or will be fungible with the old one. The CUSIP of the new note ceases to exist.
For example, after the auction of a two-year note with CUSIP 912828P61, it was found that the coupon rate and payment dates coincide with the five-year note CUSIP 912828UR9. After that, a five-year note with CUSIP 912828UR9 was surprisingly reopened.