table <str> references a system relation field <str> and can not
  be exported

  Explanation:  A table cannot be exported when  it  references  a
  system relation field

  User Action:  Remove the unsupported usage  and  try  again,  or
  compensate  for  the warning by editing the output file from the
  RMU Extract command.  The preferred  action  is  to  remove  the
  unsupported usage, even if you decide not to use the RMU Extract
  command.