Full_Description

   To specify that an empty file is to be overlaid with sorted
   records, use the /OVERLAY qualifier.

   If the input file organization is indexed-sequential, the output
   file must already exist and must be empty. If the output file
   is not empty, /OVERLAY does not write over the file. Instead, it
   appends the result of the sort to the existing output file.

   If the input file organization is sequential or relative, you can
   create an empty file for the sorted records using an OpenVMS RMS
   program and use the /OVERLAY qualifier to specify that the output
   file is to be overlaid.

   You can use the Create/FDL utility to create an empty data
   file; use the /OVERLAY qualifier to specify that Sort is to
   write output to that file. Any attributes that you specify when
   creating the empty file then become attributes of the Sort output
   file.

   See the OpenVMS Record Management Services Reference Manual for
   more information. You can also refer to this manual if you want
   to use the Convert utility to produce an indexed-sequential file
   on output.