Guest Author: Marc D. Anderson
In my jQuery Library for SharePoint Web Services, one of the most popular functions is $().SPServices.SPCascadeDropdowns. The SPCascadeDropdowns function lets you set up cascading dropdowns on SharePoint forms. What this means is that you can enforce hierarchical relationships between column values. This is sometimes called connected dropdowns or linked dropdowns (or probably other things I haven�t run across yet).
The function uses the GetListItems operation of the Lists Web Service to refresh the allowable values based on relationships which are maintained in reference lists. By implementing this function, you can remove all of the coding requirements to manage the hierarchical relationships (once it is in place) and let your users manage the content.