Choosing Legacy will continue to issue an ID in the same manner they were generated in previous version of ASP.NET, by concatenating the ID values of each parent naming container with the ID of the control.  Setting the property to Static will use the exact value of the ID property of the server control.  Predictable is used for controls that are data-bound controls such as repeater and also makes use of a ClientIDRowSuffix property.  Using Inherit makes the control ID property use the setting of its parent control.

In the example below, two lists are created inside the ContentPlaceHolder of a page using the same DataSource. The first is using the default and the second uses the new property with Static set as the value.

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
    <asp:XmlDataSource ID="LinkData" runat="server" XPath="Colors/Color">
        <Data>
            <Colors>
                <Color Name="Red"/>
                <Color Name="Blue"/>
                <Color Name="Yellow"/>
                <Color Name="Green" />
            </Colors>
        </Data>
    </asp:XmlDataSource>
    <asp:BulletedList ID="uxList" DataSourceID="LinkData" runat="server" DataTextField="Name" />
    <asp:BulletedList ID="uxListStatic" ClientIDMode="Static" DataSourceID="LinkData" runat="server" DataTextField="Name" />

    <script type="text/javascript">
        $(function() {
            $("#uxList").append("<li>Red</li>");
            $("#uxListStatic").append("<li>Brown</li>");
        });
    </script>
</asp:Content>

Here is the output of the page in HTML

<div>
    <ul id="ctl00_ContentPlaceHolder1_uxList">
        <li>Red</li><li>Blue</li><li>Yellow</li><li>Green</li>
    </ul>
    <ul id="uxListStatic">
        <li>Red</li><li>Blue</li><li>Yellow</li><li>Green</li>
    </ul>

    <script type="text/javascript">
    $(function() {
        $("#uxListStatic").append("<li>Brown</li>");
    });
    </script>
</div>

And the corresponding view in the browser

•    Red
•    Blue
•    Yellow
•    Green

•    Red
•    Blue
•    Yellow
•    Green
•    Brown

Because of the generated tag on the first list jQuery can’t find the control to append to.