When 90 days or fewer are left in the original bonds' terms, the refunding is called "current". When more than 90 days remain, the refunding is called "advance". Alternatives to a crossover refunding include net cash refunding, which is more common, and full cash or gross refunding, which is less common.
A locality might decide to refund its bonds in order to get a better interest rate, to get better debt covenants or to obtain a better debt service schedule.