Serial numbers identify otherwise identical individual units with many, obvious uses. Serial numbers are a deterrent against theft and counterfeit products, as they can be recorded, and stolen or otherwise irregular goods can be identified. Banknotes and other transferable documents of value bear serial numbers to assist in preventing counterfeiting and tracing stolen ones.
They are valuable in quality control, as once a defect is found in the production of a particular batch of product, the serial number will identify which units are affected.
Serial numbers may be used to identify individual physical or intangible objects (e.g., computer software or the right to play an online multiplayer game). The purpose and application is different. A software serial number, otherwise called product key, is usually not embedded in the software, but is assigned to a specific user with a right to use the software. The software will function only if a potential user enters a valid product code. The vast majority of possible codes are rejected by the software. If an unauthorised user is found to be using the software, the legitimate user can be identified from the code. It is usually not impossible, however, for an unauthorised user to create a valid but unallocated code either by trying many possible codes, or reverse engineering the software; use of unallocated codes can be monitored if the software makes an Internet connection.
The term "serial number" is sometimes used for codes which do not identify a single instance of something. For example, the International Standard Serial Number or ISSN used on magazines and other periodicals, an equivalent to the International Standard Book Number (ISBN) applied to books, is serially assigned not to each individual copy but to an issue of a periodical. It takes its name from the library science use of the word "serial" to mean a periodical.
Certificates and certificate authorities (CA) are necessary for widespread use of cryptography. These depend on applying mathematically rigorous serial numbers and serial number arithmetic, again not identifying a single instance of the content being protected.