They are in use by the postal machines of automatic cancellation for more than one day. That is why they can be found easier than the special commemorative postmarks. They advertise views, cultural events and, in some cases, they approach the role of the propagandistic ones (voluntary blood donation, safety from AIDS etc.). They transmit passwords of national, political or patriotic character, aiming to influence public opinion. Propaganda cancellations are usually found easily and their thematic value great.