ASP Markup:
===================
<telerik:RadMultiSelect ID="dnCategory" runat="server" DataSourceID="dsCategory" DataValueField="Category" DataTextField="Category" DropDownHeight="400px" Filter="Contains" Width="80%" Placeholder="Select Categories" RenderMode="Lightweight" TagMode="Single" CssClass="w3-round-large">
      <ClientEvents OnSelect="dnCategory_OnSelect" />
</telerik:RadMultiSelect>
<telerik:RadToolTip ID="Categorytt" runat="server" TargetControlID="dnCategory" Position="BottomCenter" RelativeTo="Mouse" Width="200px" Title="Selected Categories" ShowEvent="OnMouseOver"> 
</telerik:RadToolTip>


Client Script:MultiSelect OnSelect Event
===========================================
<script type="text/javascript">
        var selecteditems = [];
        function dnCategory_OnSelect(s, e) {
            var tooltip = $find("<%=Categorytt.ClientID%>");
            var itemvalue = e.get_dataItem().value;
                selecteditems.push(itemvalue);
            var htmlcontent = '<ol>';
            for ( var i = 0; i < selecteditems.length; i++) {
                htmlcontent = htmlcontent + '<li>' + selecteditems[i] + '</li>'
            }
            htmlcontent = htmlcontent + '</ol>'
            if (htmlcontent.length > 9) {  // html code contains more than <ol></ol> tags.
                tooltip.set_content(htmlcontent)
                varttelm = tooltip.get_contentElement();
                //tooltip.show(); not the desired recation but when used shows in top left of browser instead of the target control.
            }
        }
  </script>