VMS Help CMS, INSERT, GENERATION, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The INSERT GENERATION command places the specified element
    generation into one or more classes. The class or classes must
    already exist. (See the description of the CREATE CLASS command.)

    A class can contain only one generation of an element. You
    cannot insert any generations into a class that has the read-only
    attribute. (See the description of the MODIFY CLASS command.)