The value of this property specifies the client side ID of the element to which Easy Menu will attach (will appear).
One Easy Menu can be attached to multiple elements.
If you want to specify multiple elements, use commas to separate the IDs: ,

<div id="div1"></div>
<div id="div2"></div>
<div id="div3"></div>

<oem:EasyMenu id="EasyMenu1" runat="server" AttachTo="div1, div2, div3">
    <Components>
        <oem:MenuItem InnerHtml="Change to Red" ID="menuItem1"></oem:MenuItem>
    ...
    </Components>
</oem:EasyMenu>    

Hover these to test:

New! You can attach Easy Menu to an existing Easy Menu item to create submenus.
For example:
<oem:EasyMenu id="EasyMenu1" runat="server" AttachTo="menuItem1">
    ...


Note1: the client ID is not always the same as the server ID!
Note2: the element does not necessarily have to be an ASP.NET control. It can be a html control as well.