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

       /TRUNCATE
       /NOTRUNCATE (default)

    Command Qualifier

    Controls whether a copy or restore operation truncates a
    sequential output file at the end-of-file (EOF) when creating
    it. By default, a copy or restore operation uses the allocation
    of the input file to determine the size of the output file.