| FHIR Infrastructure Work Group | Maturity Level: 1 | Informative | Use Context: Any | 
Detailed Descriptions for the elements in the extension.
| Extension | |||||||||||||
| Element Id | Extension | ||||||||||||
| Definition | If the value is not a proper value, indicates the reason.  | ||||||||||||
| Cardinality | 0..1 This element is affected by the following invariants: ele-1 | ||||||||||||
| Comments | Considerable care must be used when using nullFlavor in this context of FHIR - more than any other concept, this is tied to the way v3 works, and FHIR is quite different. For instance, there is no notion of a "proper value" as opposed to any other value in FHIR. a NullFlavor should be understood to mean, "why information is missing", and the nulFlavors OTH, NINF, PINF, and TRC should not be used.  | ||||||||||||
| Invariants | 
  | ||||||||||||
| Extension.id | |||||||||||||
| Element Id | Extension.id | ||||||||||||
| Definition | Unique id for the element within a resource (for internal references). This may be any string value that does not contain spaces.  | ||||||||||||
| Cardinality | 0..1 | ||||||||||||
| Type | id | ||||||||||||
| Extension.extension | |||||||||||||
| Element Id | Extension.extension | ||||||||||||
| Definition | An Extension  | ||||||||||||
| Cardinality | 0..0 | ||||||||||||
| Type | Extension | ||||||||||||
| Invariants | 
  | ||||||||||||
| Slicing | This element introduces a set of slices. The slicing rules are: 
  | ||||||||||||
| Extension.url | |||||||||||||
| Element Id | Extension.url | ||||||||||||
| Definition | Source of the definition for the extension code - a logical name or a URL.  | ||||||||||||
| Cardinality | 1..1 | ||||||||||||
| Type | uri | ||||||||||||
| 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.  | ||||||||||||
| Fixed Value | http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor | ||||||||||||
| Extension.value[x] | |||||||||||||
| Element Id | Extension.value[x] | ||||||||||||
| Definition | Value of extension - must be one of a constrained set of the data types (see Extensibility for a list).  | ||||||||||||
| Cardinality | 1..1 | ||||||||||||
| Terminology Binding | null The codes SHALL be taken from http://terminology.hl7.org/ValueSet/v3-NullFlavor|4.3.0  | ||||||||||||
| Type | code | ||||||||||||
| [x] Note | See Choice of Data Types for further information about how to use [x] | ||||||||||||
| Invariants | 
  | ||||||||||||