All the options you supplied are different, you should carefully read the fix npt documentation (http://lammps.sandia.gov/doc/fix_nh.html
) and all options. The barostat keywords you are modifying allow you to set each component of the pressure tensor individually.
fix fixnpt all npt temp 298.15 298.15 2000 x 0 0 2000 y 0 0 2000 z 98.6923 98.6923 2000 couple xy drag 20.0
This set the target pressure for the x and y component of the box to 0, and the z to 98.6923. The x and y components are coupled and hence scaled by the same amount during a barostat move.
fix fixnpt all npt temp 298.15 298.15 2000 aniso 98.6923 98.6923 2000 couple xy drag 20.0
This sets the target pressure to 98.6923 for all components, otherwise same as above.
fix fixnpt all npt temp 298.15 298.15 2000 z 98.6923 98.6923 2000 drag 20.0
This tells the barostat to only make moves in the z component with a target of 98.6923.