| # | | Name | Source | Ver | Description |
| 1 | | additional identifier | hl7.fhir.uv.extensions.r4#1.0.0 | R4 | An additional identifier for the target resource. This is used when there is no way to reference the other resource directly, either because the entity it represents is not available through a FHIR server, or because there is no way for the author of the resource to convert a known identifier to an actual location. There is no requirement that a Reference.identifier point to something that is actually exposed as a FHIR instance, but it SHALL point to a business concept that would be expected to be exposed as a FHIR instance, and that instance would need to be of a FHIR resource type allowed by the reference. |
| 2 | | Alternate Reference | hl7.fhir.uv.extensions.r4#1.0.0 | R4 | Used when the target of the reference has a type that is not allowed by the definition of the element. In general, this should only arise when wrangling between versions using cross-version extensions. |
| 3 | | DEQM Criteria Reference Extension | hl7.fhir.us.davinci-deqm#current | R4 | Identifies which measure criteria (i.e. population, stratifier, or supplementalData) was responsible for the inclusion of the evaluatedResource or supplementalData on which it appears. |
| 4 | | DEQM Population Reference Extension | hl7.fhir.us.davinci-deqm#current | R4 | Deprecated: Identifies which measure population that a specific resource is used for evaluating their logics by referencing population criteria id (`Measure.group.population.id`) specified in Measure resource. |
| 5 | | DEQM Reporting Program Extension | hl7.fhir.us.davinci-deqm#current | R4 | Extension that supports the reporting program as a reference to a Library conforming to the Quality Program profile, as well as a CodeableConcept bound to the Quality Program value set. |
| 6 | | Resolve as version specific | hl7.fhir.uv.extensions.r4#1.0.0 | R4 | If true, indicates that the reference should be resolved to a version-specific reference rather than a version-agnostic reference. |
| 7 | | Target element | hl7.fhir.uv.extensions.r4#1.0.0 | R4 | Indicates that the reference has a particular focus in the target resource, a particular element of interest, identified by it's element id. |
| 8 | | Target path | hl7.fhir.uv.extensions.r4#1.0.0 | R4 | Indicates that the reference has a particular focus in the target resource, a particular element of interest, identified by a FHIRPath statement. The FHIRPath expression is limited to a the [simple subset](fhirpath.html#simple) with the additional limitation that .resolve() that is not allowed. |