sort key is external to RSE context

  Explanation:  A sort key refers to record stream  contexts  that
  are  totally  external  to  the Record Selection Expression that
  contains the sort clause.  This type of  sort  key  provides  an
  invariant  value to the sort, making the sort more costly for no
  useful purpose, and may indicate an improperly  positioned  sort
  clause.

  User Action:  Define the sort key or keys again,  or  reposition
  the sort clause to a more external RSE.