Specifies whether SQL should inherit roles from the operating
    system. If you specify one of these clauses, you must specify
    the same clause as was specified when the role was created.
    You cannot use the ALTER ROLE statement to change roles from
    IDENTIFIED EXTERNALLY to NOT IDENTIFIED or from NOT IDENTIFIED to
    IDENTIFIED EXTERNALLY.

    The IDENTIFIED EXERNALLY clause indicates that SQL inherits the
    roles defined by the facilities of the operating system, such as
    OpenVMS rights identifiers.

    The NOT IDENTIFIED clause indicates that SQL does not inherit any
    roles defined by the facilities of the operating system; instead,
    the role is private to the database.