The Map Parameters Tab.
This very important screen handles the information stored in
the world_gen.txt file and also allows you to set which maps should be
generated by PerfectWorldDF. To view and edit these fields, first you
must import your world_gen.txt file from your df_xx_xx\data\init folder
by selecting the File menu and selecting 'Import world_gen File'. To
save any changes you make to the world_gen.txt file, you must select
the File menu again and select "Export world_gen File'. Nothing is ever
written to the world_gen.txt file automatically.

The 'Parameter Set' group box contains controls to add and remove
parameter sets and also to change the map sizes. To select a parameter
set to view, use the list box labled 'Name:' and select the desired
parameter set name from the list.
To add a new parameter set, select a param set that you want to copy
and then type a new name into the list box and press the 'Enter' key.
This will create a new param set called by your new name.
To delete a parameter set, first select the param set you wish to
delete, and then press the 'Delete Param Set' button.
You may change the map size for the selected parameter set, but this
will delete any pre-set map data that was stored in the world_gen.txt
file. Also, many fields in the param set (marked in light green)
will be automatically resized by PerfectWorldDF. This should be
desireable in most cases but not necessarily. Have another look at the
green fields if you resize the map in this way to make sure they are
reasonable.
To use the maps generated in PerfectWorldDF you must select the check
boxes for each map you want to generate. This action will erase some of
the map rejection criteria fields and mark them in red. You may go back
and change those values but usually you don't want this rejection
criteria for PW generated maps. Any pre-set map data stored in the
param set before you selected the map for PW generation will be
overwritten upon exporting to the world_gen.txt file.
For information regarding the individual parameter fields inside of
world_gen.txt, most of what you need can be found in the Dwarf Fortress Wiki.