Diff operation
Use the $diff operation to generate a differential between two versions of a resource, or between two different resources of the same type.
Differentials generated by this operation are in FHIR Patch format.
In generated differentials, where a value has changed (that is, a replace operation), an additional part value is present on the given operation called previousValue. This part shows the value as it was in the from version of the resource.
Parent topic: Reading data