VMS Help INSTALL, ADD, Qualifiers, /PURGE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /PURGE (default)
       /NOPURGE

    Specifies that the image can be removed by a purge operation; if
    you specify /NOPURGE, you can remove the image only by a delete
    or remove operation.