/sys$common/syshlp/helplib.hlb MERGE, Output File Qualifiers, /ALLOCATION, Full Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Sort/Merge preallocates space for the output file based on total
    input file allocation, thereby avoiding the overhead of extending
    the file every time another few blocks are written to it.

    However, if you know that the output file allocation will differ
    substantially from the total input file allocation (because you
    are reformatting data or omitting records), you can specify the
    number of blocks to be preallocated for the output file.

    The /ALLOCATION qualifier is required if the /CONTIGUOUS
    qualifier is used.