This is called a conditional value list:
Here are some useful links on the subject:
Forum Tutorial: Custom Value List?
And if you are interested in Enhanced Value list techniques that make it possible to find products by entering partial text from the description field into an auto-complete enabled drop down list while still maintaining the correct ProductID based relationships, see this demo file: http://www.4shared.com/file/plr_jbkk/EnhancedValueSelection.html
thank you again phil you rule ....