Unique id for inter-element referencing
Unique id for the element within a resource (for internal references). This may be any string value that does not contain spaces.
Additional content defined by implementations
May 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.
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.
The low limit. The boundary is inclusive.
If the low element is missing, the low boundary is not known.
The high limit. The boundary is inclusive.
If the high element is missing, the high boundary is not known.