Clients of a hypermedia REST API are supposed to have a priori knowledge of the link relations (the "rel" attribute). The fact the rel in this example looks li

Clients of a hypermedia REST API are supposed to have a priori knowledge of the link relations (the "rel" attribute). The fact the rel in this example looks like a URI is sometimes used as a convention (i.e., if the API supports it, a client may do a GET against the rel value, and get back a human-readable description of the purpose of the rel). In the human readable description of the "/linksrels/slot/book" link relation, information should be given about what generally can be done with such links (like if GET or POST is supported, etc). This allows you to build your client applications. The value of the actual link (the 'uri' attribute) should be completely opaque to the client.