Full_Description

   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.