/sys$common/syshlp/helplib.hlb CMS, RESERVE, Qualifiers, /GENERATION[=generation-expression]  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Specifies a particular generation of the element that is to be
    reserved. If you omit /GENERATION, CMS reserves the most recent
    generation on the main line of descent.