White spaces or white spots

White spaces or white spots are empty pieces of spectrum scattered between used frequencies. There can be various reasons for the fact that the spectrum is not used. It can be that the frequencies are assigned to a user, but at a particular time and specific geographic location, not used by that user, e.g because there is no demand from customers. It can also be that the frequencies are not used because they serve as a guard band between various transmitters or services.