Precancels
Any stamp that is cancelled by postal authorities before it is sold is described as "precancelled": the precancellation mark is an overprint. This is usually only done when stamps are sold in large bulk quantities to businesses or other large organizations: the postal service will save the labor of cancelling each individual stamp by precancelling the entire purchased quantity. The overprints also help prevent theft or misuse because they usually include the name of the city or region in which they are to be used. Unlike standard cancellation marks, they usually do not give a specific date, affording the bulk purchaser time to use them at their discretion. In some situations, however, months or years may be included in the overprint to indicate an expiration.
Precancels for official government use are fastidiously prepared, but other kinds are almost always "heavy cancels" which deliberately obliterate much of a stamp's design. For this reason, precancelled stamps are frequently unrepresented in philatelic collections.
Since the 1980s, many modern postal systems no longer use overprints to indicate bulk purchases. Bulk mail is franked using barcodes on pre-printed envelopes or on blank adhesive labels. The USPS introduced a new standard of barcode cancellation in 2011.
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Canada, 1870: Early heavy cancel for bulk purchase.
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France, 1920: Bulk postage precancel marked for Paris.
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USA, 1938: Business precancel marked for New York City.
Read more about this topic: Overprint