Verifies that the value of the field is unique throughout the application. For example, suppose displayName is marked as a unique field, and a user tries to set August Joseph Springer as their display name. If another user is already has August Joseph Springer as their display name then validation fails. There can only be one August Joseph Springer.
Note that uniqueness applies only to a given field: a user could have a different field (and a different user profile attribute) set to August Joseph Springer.
The unique validation can be used by the following field types: