| There are no (further) constraints on this elementElement idShort description Additional content defined by implementationsAlternate names extensions, user contentDefinition CommentsMay be used to represent additional information that is not part of the basic definition of the element. 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 can define an extension, there is a set of requirements that SHALL be met as part of the definition of the extension.Data typeThere 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.ExtensionSliced: Unordered, Open, by url(Value) Extensions are always sliced by (at least) urlConstraints Mappingsele-1: All FHIR elements must have a @value or children
 hasValue() or (children().count() > id.count())ext-1: Must have either extensions or value[x], not both
 extension.exists() != value.exists()
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