A unique device identifier (sometimes called a universally unique ID or UUID) is a string of characters that is incorporated into a device by its manufacturer and can be used to uniquely identify that device (for example an IMEI-number of a mobile phone). Different device identifiers vary in how permanent they are, whether they can be reset by users, and how they can be accessed. A given device may have several different unique device identifiers. Unique device identifiers can be used for various purposes, including security and fraud detection, syncing services such as a user’s email inbox, remembering the user’s preferences and providing relevant advertising.