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

       /TRUNCATE
       /NOTRUNCATE

    Controls whether COPY truncates an output file at the end-of-
    file when copying it. The default is /NOTRUNCATE; COPY uses the
    allocation of the input file to determine the size of the output
    file.