This is Dave with Bullhorn Support.
While we do not have a UML to forward, you can utilize the Entity Reference Guide included in the document linked below.http://bullhorn.github.io/rest-api-docs/
After opening the document, go to "Other Guides: - Entity Reference Guide available: here"
This will provide insight on utilizing meta calls for the entities.
Please let us know if you do have any additional questions or concerns.