The Derived-From entity-header field can be used to indicate the version tag of the resource from which the enclosed entity was derived before modifications were made by the sender. This field is used to help manage the process of merging successive changes to a resource, particularly when such changes are being made in parallel and from multiple sources.
Derived-From = "Derived-From" ":" quoted-string
An example use of the field is:
The Derived-From field is required for PUT and PATCH requests if the entity being sent was previously retrieved from the same URI and a Content-Version header was included with the entity when it was last retrieved.