> ## Documentation Index
> Fetch the complete documentation index at: https://docs.metriport.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Extension

> Base StructureDefinition for Extension Type: Optional Extension Element - found in all resources.

## Properties

| Name | Required | Type | Description |
| - | - | - | - |
| id | No | string | Unique id for inter-element referencing |
| extension | No | [Extension](/medical-api/fhir/data-types/extension)\[] | Additional content defined by implementations |
| url | Yes | string | identifies the meaning of the extension |
| value\[x] | No | base64Binary, boolean, canonical, code, date, dateTime, decimal, id, instant, integer, markdown, oid, positiveInt, string, time, unsignedInt, uri, url, uuid, [Address](/medical-api/fhir/data-types/address), [Age](/medical-api/fhir/data-types/age), [Annotation](/medical-api/fhir/data-types/annotation), [Attachment](/medical-api/fhir/data-types/attachment), [CodeableConcept](/medical-api/fhir/data-types/codeableconcept), [Coding](/medical-api/fhir/data-types/coding), [ContactPoint](/medical-api/fhir/data-types/contactpoint), [Count](/medical-api/fhir/data-types/count), [Distance](/medical-api/fhir/data-types/distance), [Duration](/medical-api/fhir/data-types/duration), [HumanName](/medical-api/fhir/data-types/humanname), [Identifier](/medical-api/fhir/data-types/identifier), [Money](/medical-api/fhir/data-types/money), [Period](/medical-api/fhir/data-types/period), [Quantity](/medical-api/fhir/data-types/quantity), [Range](/medical-api/fhir/data-types/range), [Ratio](/medical-api/fhir/data-types/ratio), Reference\<>, [SampledData](/medical-api/fhir/data-types/sampleddata), [Signature](/medical-api/fhir/data-types/signature), [Timing](/medical-api/fhir/data-types/timing), [ContactDetail](/medical-api/fhir/data-types/contactdetail), [Contributor](/medical-api/fhir/data-types/contributor), [DataRequirement](/medical-api/fhir/data-types/datarequirement), [Expression](/medical-api/fhir/data-types/expression), [ParameterDefinition](/medical-api/fhir/data-types/parameterdefinition), [RelatedArtifact](/medical-api/fhir/data-types/relatedartifact), [TriggerDefinition](/medical-api/fhir/data-types/triggerdefinition), [UsageContext](/medical-api/fhir/data-types/usagecontext), [Dosage](/medical-api/fhir/data-types/dosage), [Meta](/medical-api/fhir/data-types/meta) | Value of extension |


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