Bonds can be called for any other reason than refunding
For example ,Consider company has $1000 bond at coupon rate 8% with market rate of interest at 7%. Now imagine market falls to 6% So if the company has call option with its bond, it can call all it bonds and refund them at lower coupon rate of 6% leading to huge saving. So the refunding bonds are the ones for which bonds can be called but not refunded.