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relationshipOfResource (http://rs.tdwg.org/dwc/terms/relationshipOfResource)

@return [String]
The relationship of the resource identified by relatedResourceID to
the subject (optionally identified by the resourceID). Recommended
best practice is to use a controlled vocabulary.

Example: "duplicate of", "mother of", "endoparasite of", "host to",
"sibling of", "valid synonym of", "located within"
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