The chain select combobox is a usaual need in a web project. Like a Combo box population based on another combobox change event. For example a city combo is populated on change of state combo. Subcategory combo is populated in change of category combo. The select boxes in HTML forms are flexible and can be handled by jquery system.
Ziadin Givan written a nice plugin for chained combo box select and complete. also there is a jquery plugin from Remy Sharp which is called Select Chain Both are great plugins for this feature.