The information that a ResourceProxy can contain (a URL and mimetype) is kept very minimal, on purpose. However you can use any component to add more details about such a ResourceProxy, using the id attribute.
E.g. in the example CMDI file we can add a textual description of the photo. First the relevant ResourceProxy gets the id "a_photo":
<ResourceProxy id="a_photo"> <ResourceType mimetype="image/jpeg">Resource</ResourceType> <!-- note that both a normal URL and a handle Persistent Identifier can be used for the ResourceRef --> <ResourceRef>hdl:1839/00-0000-0000-0009-3C7E-F</ResourceRef> </ResourceProxy>
Then, later on in the same CMDI file, we have an explanantory component example-component-photo with a description element:
<example-component-photo ref="a_photo"> <description>a suitable textual description of this photo</description> </example-component-photo>
Thanks to the reference from this component to the ResourceProxy with the ref attribute we know that the description relates to the photo.
Note that the id attribute should be unique for each ResourceProxy.