/sys$common/syshlp/helplib.hlb NCS Command, Command Qualifiers, /DATA  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  /DATA=option

   This qualifier gives you additional capabilities for obtaining  disk
   space efficiency. Note that you must specify an  option  with  this
   qualifier, either the REDUCE option or the EXPAND option. Note too,
   that NCS does an implicit compression (recovers unused  space  from
   previously   deleted   files)  on  the  library,  whether  you  are
   converting the library to reduced form or to expanded form.

   If your NCS library is in standard  (non-reduced)  form,  use  this
   qualifier with the REDUCE option to create a new reduced version of
   the library.  If you do not use the /LIBRARY qualifier to specify a
   library, NCS  creates  a  new  reduced  version of  NCS$LIBRARY.NLB
   in SYS$LIBRARY.  Note that access to libraries in reduced format is
   generally slower.

   If your NCS library is in reduced form, use this qualifier with the
   EXPAND option to create a new expanded (standard)  version  of  the
   library.  If you do not use the /LIBRARY  qualifier  to  specify  a
   library, NCS creates a new, expanded version of  NCS$LIBRARY.NLB in
   SYS$LIBRARY. Note that access to libraries in  expanded  (standard)
   format is generally faster.

   NCS provides no default values for this qualifier.  You must select
   one of the two options.