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

       /START_BLOCK=[n]

    For RT-11 volumes, specifies the logical block number where the
    file is to be placed. This qualifier is especially useful with
    TU58 tape cassettes, because performance can be significantly
    enhanced by careful placement of files.