We use this feature in a few places. One example is that we wanted to prefill several fields such as Priority, Impact, and Assignment;Workgroup based on the Classification that the user picks. So, we created a database with all the classifications and the other values.
The external system comes in where we created a Perl script on a web server -- it accepts the classification as input, looks up the other values in our separate database, and then returns the values as output. Service Desk uses those output values to UPDATE the fields specified.
You can find more detail about what format the values need to be in and such in the HP Admin Guide. To summarize the feature though, it allows you to call an external script/program to do some function for you -- this is useful when Service Desk alone can't perform a given task.
Hope that clears it up a bit. Maybe someone else can chime in with their experience as well.
Attached you will find a boiled down version of the Perl. It's pretty straightforward. The arguments come in from Service Desk as output_field_value_0, output_field_value_1, etc... You can see the code to retrieve those parameters at the top under the variable "@names". To send values back to Service Desk, you use the format "n_fields=[number_of_fields_passing_back]&field_0=[first_value]&field_1=[second_value]..." You can see that syntax near the bottom of the file.
Eric, if the example mentioned above in conjunction with this Perl doesn't help you, then I really can't advocate much more than to check out the admin guide and lookup the section titled "addendum to SP8". I believe that's where the details on this feature can be found. Here's a link to the PDF on HP's site (see appendix B): http://ovweb.external.hp.com/ovnsmdps/pdf/sd45_sp8_addendum_eng.pdf
Maybe someone else has a clearer/simpler example then?