What is "postal fiscal stamp"

A postal fiscal is a revenue stamp that has been authorised for postal use. Postal fiscals may arise because there is a shortage of postage stamps for a country or out of economy to use up obsolete or excess stocks of revenue stamps. Postal fiscals are to be distinguished from stamps marked "Postage and Revenue" which were always intended for either use, or revenue stamps used postally by accident or because local postal regulations did not prohibit such use. Postal fiscal status may usually only be identified from the cancels on used stamps or where the stamp is found on cover.