This is the Continuous Integration Build of FHIR (will be incorrect/inconsistent at times). 
See the Directory of published versions 
| FHIR Infrastructure  Work Group | Maturity Level: Normative | Standards Status: Normative | 
| Extension | |||||||||
| Standards Status | This element has a standards status of "Normative" | ||||||||
| Definition | Optional Extension Element - found in all resources. | ||||||||
| Short Display | Optional Extensions Element | ||||||||
| Cardinality | 0..* | ||||||||
| Type | DataType | ||||||||
| Requirements | The ability to add extensions in a structured way is what keeps FHIR resources simple. | ||||||||
| Constraints | 
 | ||||||||
| Extension.url | |||||||||
| Definition | Source of the definition for the extension code - a logical name or a URL. | ||||||||
| Short Display | identifies the meaning of the extension | ||||||||
| Cardinality | 1..1 | ||||||||
| Type | uri | ||||||||
| Summary | true | ||||||||
| Comments | The definition may point directly to a computable or human-readable definition of the extensibility codes, or it may be a logical URI as declared in some other specification. The definition SHALL be a URI for the Structure Definition defining the extension. | ||||||||
| Extension.value[x] | |||||||||
| Definition | Value of extension - must be one of a constrained set of the data types (see Extensibility for a list). | ||||||||
| Short Display | Value of extension | ||||||||
| Cardinality | 0..1 | ||||||||
| Type | * | ||||||||
| [x] Note | See Choice of Datatypes for further information about how to use [x] | ||||||||
| Summary | true | ||||||||
| Constraints | 
 | ||||||||