VMS Help BACKUP Command, Qualifiers, /COMPARE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Command Qualifier

    Causes BACKUP to compare the contents of the first parameter
    with the contents of the second parameter. The first parameter
    can be a Files-11 file, a wildcard character representing a
    group of files, a BACKUP save set, or a disk device. The second
    parameter must be a Files-11 disk file or files, or a Files-11
    disk device. Unless you also specify the /PHYSICAL qualifier, the
    disk volume must be a Files-11 volume. If a file does not compare
    successfully, BACKUP displays an error message.