Put it this way. As is pointed out in the other comment a Bond is not considered currency and states issue those. A Bond in is just a certificate of debt. Whoever issues the bond is borrowing money from you and then paying you interest in return. So it represents currency but is not in itself currency.
An IOU is simply a Bond that doesn't collect interest. California is in effect using it's governing power to force it's vendors to buy high-risk, no interest Bonds from them.