How to build vegetation on a terrain
An example based on Baro's experience.
First of all, get hold of the world's elevdump either by a bot or by
the admin tool elevdump utility. Make sure the elevdump size is
symmetric to the center of the world.
In our example it was NOT symmetric and it would add several steps to the vegetation generating.
I queried the terrain from the world with the YASBB utility (1) and re saved with the "Save Edited Elev Dump" button (2) which makes it a square instead of a rectangle for further processing:

Next step is starting Dem2RWX and loading the newly saved elevdump (3):

Don't forget to set the file type to "AW Elevation Dump".

We want to build objects, so we should instruct the program to respect the world size (5) (building beyond the world boundary gives us error message from the world server!)
Since we use the elevation dump as our input, we have to make the
proper center location available for the program. The elevation grid
points are on a different location than the cell center (usually 0.5S
0.5E offset is necessary). Use the Preset Terrain in AW button (4) for setting it (just accept the offered default location in our case):

One can view the Elevation Map by clicking on the "Show Full DEM Picture" button:

In the next step, select the World Parameters tab. Make sure all
checkboxes are unchecked and set the Vertical Scaling to 0.01 because
the elevation dump is in centimeters and we want to convert it to
meters (7). On the same page set the
Scaling to 1:3 (10m) because the grid is 10 meter in AW and select the
world size you want to operate on (in our example it is a P50 world):

Start to experiment with the zones in the RWX Texture Zones tab. Each
zone assigns a different color so you can see where do you want to
plant any vegetation. Set the elevation Low-High ranges in the original
units of the elevdump (it is centimeter in this case!) (8) and click on the Show Zones button to see the result:

Here is the result of our example: red is the peaks of the terrain
(from elevation 60m to 70m), we'll have trees and plants on the blue,
and only plants on the white area:

Disable the RWX object generating feature on the RWX Option tab:

So far we are happy with the zones, so let go to the Tree Zones tab and
specify all the features we want for the vegetation or hunt objects.
First, select Generate Trees and Trees Only (9)
because we don't want to create the terrain itself, only to plant a few
objects. We want to put trees from the elevation range 34m to 70m, have
the flora20.rwx bush from 32m to 70m and have an additional flora19.rwx
plant only on the peaks from 65m to 70m.
Put the elevation ranges into the Lowest and Highest fields in centimeters (the original dem file's units!) (10).
Provide the model name's prefix on the next field. In the first zone we
want to build btree01.rwx, btree02.rwx and btree03.rwx models randomly
picked by the program, so we set the prefix to btree0, the numbering
starts from 1 (Suffix From field) and we have 3 different models (# of
Trees field). Enable Zone 1 for generating trees in this zone. We
wanted to look more realistic, so we asked the program to randomly
rotate the object. We did not want to build a too homogeneous forest, so
we selected a random object picking from our range (11).
(One can have the program to gradually pick the objects from the range
based on the elevation within the zone to simulate the real vegetation
as the weather is colder on the higher elevation). We set the Height
Offset to -2 centimeter, so the program will sink the tree slightly
into the ground. If the Random Object selected, then the Height Offset
is randomly modulated between 0 and the given range to provide more
realistic positioning.
The second and third Zone using only a single model, so you have to
specify the full model name in the RWX Name Prefix field. It will be
flora20.rwx and flora21.rwx . If you set the # of Trees to 1, the
program will always use the RWX Name Prefix for the full model name (by
appending ".rwx" to it).
For those who wants to hide eggs for an egg hunt, the program provides the following fields/switches (12):
Optional Action Line content: - set it to ";score=" without
quotes. Select the "Score" checkbox for those zones you want the
program automatically generating random score values from ScoreMin till
ScoreMax. That value will be appended to the action field effectively
generating: ";score=7" e.g. If you set the action field to
"activate ;score=" then the mouse will turn into a hand while hovering
over the object.
Each Zone will provide one object in each cell. If you want more
objects per cell, then create identical zones for them. If you think
the number of object placed in a cell it too much, use the "Build
objects on every...th unit" feature to reduce the number.

Phew - that was a long paragraph :)
Set the Log options if you wish to log some events by enabling it in the Log options tab (14):

Now the building part!
Set the Citizen number for the OWNER of the objects built by the bot (16) If the bot builds under an Eminent Domain or a CT, it can put different ownership on the objects.
Select the prop file location if you want to have the propfile generated.
If you have Terrain Offset specified in the World Options, put the same
value (in centimeters of course) into the Prop Elevation field (15)
! As you can see, the program already filled in the Prop Center offset
with the Preset Terrain Center button from the Elevation tab.
Fill in the bot credentials and select the Best Robot Building speed
based on your connection and the world server's performance (one notch
below max is usually adequate for an average world server)

Double check all settings again and if you are happy with them, hit the
Generate button. You can stop the bot while it is building by clicking
on the "STOP!! I changed my mind" button.
In case you hit exit, you'll greeted with this dialog:

Thats all folks!