I don't really know exactly how it's supposed to work. It may not be a visible field that you directly type in. I think it requires a relatively sophisticated user interface where the user would highlight a region within the image and then attach a name to that region and the software then codes it into XMP in a way that looks like this:
...
<mwg-rs:Regions rdf:parseType=“Resource”>
<mwg-rs:AppliedToDimensions stDim:w=“4288” stDim:h=“2848” stDim:unit=“pixel”/>
<mwg-rs:RegionList>
<rdf:Bag>
<!-- Simple example for face detection -->
<rdf:li rdf:parseType=“Resource”>
<mwg-rs:Area stArea:x=“0.5” stArea:y=“0.5” stArea:w=“0.06” stArea:h=“0.09” stArea:unit=“normalized”/>
<mwg-rs:Type>Face</mwg-rs:Type>
<mwg-rs:Title>John Doe</mwg-rs:Title>
</rdf:li>
<!-- Simple example for pet detection -->
<rdf:li rdf:parseType=“Resource”>
<mwg-rs:Area stArea:x=“0.5” stArea:y=“0.5” stArea:w=“0.06” stArea:h=“0.09” stArea:unit=“normalized”/>
<mwg-rs:Type>Pet</mwg-rs:Type>
<mwg-rs:Title>Fido</mwg-rs:Title>
<mwg-rs:Description>Fido looks happy!</mwg-rs:Description>
</rdf:li>
<!-- Metadata applied to a specific region, defined locally -->
<rdf:li rdf:parseType=“Resource”>
<mwg-rs:Area stArea:x=“0.5” stArea:y=“0.5” stArea:w=“0.003” stArea:h=“0.002” stArea:unit=“normalized”/>
<mwg-rs:Type>Focus</mwg-rs:Type>
<mwg-rs:FocusUsage>EvaluatedUsed</mwg-rs:FocusUsage>
</rdf:li>
<rdf:li rdf:parseType=“Resource”>
<mwg-rs:Area stArea:x=“0.5” stArea:y=“0.5” stArea:w=“0.003” stArea:h=“0.002” stArea:unit=“normalized”/>
<mwg-rs:Type>BarCode</mwg-rs:Type>
<mwg-rs:BarCodeValue>ISBN:1234567890</mwg-rs:BarCodeValue>
<mwg-rs:Name>The Best Book</mwg-rs:Name>
<mwg-rs:Description>The best book is the best book ever.</mwg-rs:Description>
</rdf:li>
</rdf:Bag>
</mwg-rs:RegionList>
</mwg-rs:Regions>