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

    The PURGE command deletes all known file entries for images
    installed without the /NOPURGE qualifier.

    If a process is accessing global sections when the PURGE command
    is entered, the global sections are deleted only after the
    operation initiated by the process completes. However, once the
    command is entered, no additional processes can access the global
    sections because they are marked for deletion.