FHIR Release 3 (STU) Current Build
FHIR Infrastructure Work Group Maturity Level : 5 Ballot Status : Trial Use Normative

Normative Candidate Note: This page is candidate normative content for R4 in the Infrastructure Package . Once normative, it will lose it's Maturity Level, and breaking changes will no longer be made.

Definition Base definition for all elements in a resource.
Element BackboneElement
Control Standards Status 1..1 Invariants Defined on this This element ele-1 : All FHIR elements must have has a @value or children ( expression : hasValue() | (children().count() > id.count()), xpath: @value|f:*|h:div) Affect this element Element.id standards status of "Normative" which is different from the status of the whole resource

unique id Base definition for the element within all elements that are defined inside a resource (for internal references). This may be any string value that does - but not contain spaces. those in a data type.

Control 0..1 1..1
Type string Element
Element.extension BackboneElement.modifierExtension

May be used to represent additional information that is not part of the basic definition of the element. In order to element and that modifies the understanding of the element in which it is contained. Usually modifier elements provide negation or qualification. To make the use of extensions safe and manageable, there is a strict set of governance applied to the definition and use of extensions. Though any implementer is allowed to can define an extension, there is a set of requirements that SHALL be met as part of the definition of the extension. Applications processing a resource are required to check for modifier extensions.

Modifier extensions SHALL NOT change the meaning of any elements on Resource or DomainResource (including cannot change the meaning of modifierExtension itself).

Control 0..*
Type Extension
Is Modifier true (Reason: Modifier extensions are expected to modify the meaning or interpretation of the element that contains them)
Alternate Names extensions; user content content; modifiers
Summary true

There can be no stigma associated with the use of extensions by any application, project, or standard - regardless of the institution or jurisdiction that uses or defines the extensions. The use of extensions is what allows the FHIR specification to retain a core level of simplicity for everyone.