UUID

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UUID is an abbreviation for Universally Unique Identifier. It is a 128-bit (16 byte) value which is generated in such a way as to make collisions very unlikely. They are often represented as a string of 32 hex characters with four dashes interspersed. See Wikipedia on UUIDs for more information.

In LSL they are stored in the key variable type (which is a specialized form of a string).