PlaceHolder Web Server Control Overview
TheWeb server control enables you to place an empty container control within the page and then dynamically add, remove, or loop through child elements at run time. The control only renders its child elements; it has no HTML-based output of its own.
As an example, you might want to have a variable number of buttons appear on a Web page, depending on options selected by users. That way, users are not confronted with potentially confusing choices that are either unavailable or not relevant to their individual needs.