This example uses an ID, but you might, for example, use a CSS class, or the element hierarchy - whatever works, so long as it uniquely identifies the right element.
Note
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{{<form>}} Form elements and the Image Reorderer
Currently the Image Reorderer is restricted to using <form> elements as its root container. This is a known issue and it is expected that in the future any container can be used as long as it is properly identified. For more information see (FLUID-4019).