Outlining is a dynamic method for displaying multilevel lists. When a list has outlining enabled and an item in the list has a sub-list indented below it, the sub-list is not displayed unless the user clicks on the list item above it. Clicking on the list item a second time hides the sub-list. By letting users expand and contract a list, you make the list more interactive and also save space in the Web browser window.
You apply outlining to a list by selecting any item in the list and choosing List Properties from the shortcut menu. In any tab of the List Properties dialog box, click Enable Collapsible Outlines. Outlining is applied to the list containing the currently selected item along with any sub-lists of the list.