Question: Where does the 'relation': 'to-many' and 'relation': 'to-one' come from=

Hello,

nice project. However, I have one question: in your schema for posting resources you use  _'relation': 'to-many' and 'relation': 'to-one'_ . Are these part of the json schema specification (and I can not find them :-)) or is this a project-specific extension of json schema, or is there another source?

Thanks for any clarification.
Markus