This parameter is NOT passed when no document information is available, e.g. when:
creating new form data
listing form data on the summary page
The value of the parameter is the id of the form data (document) being edited. This allows the persistence layer to retrieve the form definition associated with that document, in case the persistence layer handles versioning itself.
In addition to explicit versioning of form definitions, the relational database implementations support an auditing trail, keeping older versions of form definitions and form data. However this is not visible at the level of the REST API.