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

       /ALLOCATION=n

    Forces the initial allocation of the output file to the number
    of 512-byte blocks that you specified as n. The /ALLOCATION
    qualifier is valid only for Files-11 and RT-11 output files.

    By default, COPY determines the initial allocation of the output
    file by the size of the input file. Typically, /ALLOCATION is
    needed only when you are creating a contiguous file on Files-11
    (using /BEST_TRY_CONTIGUOUS or /CONTIGUOUS), when the input file
    is on magnetic tape, or when you want additional space at the end
    of the file.

    If you specify /ALLOCATION, the file's allocated size does not
    change, unless you also specify /TRUNCATE. When you are unsure of
    the output size, you might want to specify both /ALLOCATION and
    /TRUNCATE.