This is the set of resources that conform to either of the profiles Serial Number Observation (http://hl7.org/cda/us/ccda/StructureDefinition/SerialNumberObservation) and Serial Number Observation (http://hl7.org/cda/us/ccda/StructureDefinition/SerialNumberObservation). E.g. what you have to deal with if you get resources conforming to one of them