Assuming the DocumentReference.subject is a patient, can somebody explain the use case/s to convey the DocumentReference.context.sourcePatientInfo element in the DocumentReference resource . I see the definition states ‘the Patient Information as known when the document was published. May be a reference to a version specific, or contained.’, but I am still not clear when this element should be used. Would appreciate a steer with this?

This is an idea that comes out of XDS - you have two sets of patient information: the current master patient information, but for integrity, the patient information as presented in the document itself. For further information, see the XDS documentation