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A UUID is a Universally Unique Identifier, but UUIDs are not perfectly unique because no central registry is used to check whether a given ID has been used or not. Instead, UUID's are really long (128 bits), and when they are generated their bits are comprised of highly subjective or time-dependent information and/or a cryptographically secure random number sequence. In this way, the likelihood that two UUIDs are identical would be tiny even if millions of UUIDs are used in the same context.