Active Select

The active select module defines the activeselect form element. An activeselect element is the same as a regular select element, except that when the user selects a new option (or set of options), one or more different select elements (the target elements) get their lists updated. This is done using AJAX, and it is designed to degrade gracefully if the required JavaScript support is not present. The target elements can be either regular select boxes, or other activeselect boxes (which in turn can trigger other target boxes, which can trigger yet others, resulting in a hierarchical cascade of activeselect elements).

Don't bother installing this module unless some other module instructs you to do so.