You use the
http://livedocs.macromedia.com/flash/8/main/00003172.html to
respond to the choice. There is a good example at the link in the last sentence.
As far as what you do in the change handler depends on your data:
1. You can download all that data when you download the items for the combo
box. You can store the values related to the item in the combobox in the data
parameter. See
http://livedocs.macromedia.com/flash/8/main/00003170.html. For
example: comboBoxInstance.addItem({label:label[, data:data]}). Then using
http://livedocs.macromedia.com/flash/8/main/00003195.html you can get the data
and populate other items in the change event handler.
2. You only have key values stored as data in the combobox item. Then in the
change handler you go to the server and fetch the data selected and then
populate the other objects.
The first way is preferable if the amount of data is reasonable because it
requires downloading all the needed data for all the items all at one time. The
second way is necessary if the data is excessive to be downloaded all at once.