Overview#Link Relation Types is defined in RFC 5988 as an IANA Registry which can be found at: Link Relations
Link Relation Types in the simplest case identifies the semantics of a link. For example, a link with the relation type "copyright" indicates that the current link context has a copyright resource at the link target.
Link Relation Types can also be used to indicate that the Target Resource has particular attributes, or exhibits particular behaviours; for example, a "service" link implies that the link target can be used as part of a defined protocol (in this case, a service description).
Links and operations are consumed by machines who understand their meaning because they are standardized. These names are usually defined in human readable documentation, such as the one accompanying the API, the IANA Registry Link Relations or schema.org.
As the usage of schema.org increases through it being the agreed upon vocabulary for the Structured data initiatives of Google, Microsoft, Yahoo and Yandex for their search engines, its usage is surely going to increase for APIs as well. With a few standardized vocabularies and a few standardized hypermedia formats supporting them (such as Hydra), clients can stop being unique snowflakes coupled to specific APIs and instead move in the direction of generality and re-usability.