As a general rule always a navigable URL will be found in a "href", never a URL will be provided as a value for a model property. In the API model the properties that are simple references are marked with a special type "Reference". The "href" can point internally for instance in relations to other models, or externally in case the content is kept on a different server and for instance in case of a NewsML resource item you might want to provide the reference to a youtube video file. So to answer you question "href" is generated in case of model relations but when is a pointer to a different external resource is left as it is.