A Nil UUID, also known as an Empty UUID or Null UUID, is a universally unique identifier (UUID) that has a specific value of 16 bytes, which is considered "null" or "empty". The value of the nil UUID is "00000000-0000-0000-0000-000000000000", which means that it does not contain any unique information or data.
Empty UUIDs can be used in situations where a unique identifier is required, but the actual value of the identifier has not yet been determined or is not yet available. Empty UUIDs can also be used to temporarily replace an actually generated unique identifier, but which is unavailable for some reason.
In summary, the Nil UUID is a special type of UUID that has a null or empty value, and which can be used temporarily or as a substitute for a real unique UUID.