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https://www.hl7.org/fhir/STU3/structuredefinition.html
description
Used to indicate the type of a participant within an Appointment resource where the Reference contains only a display information (and so the resource type is not clear) and the version is STU3 (so R4’s Reference.type is unavailable).
A value of “Location” will indicate to PKB that the Location should be used, but otherwise this extension is not needed.
context
This extension is used by the following resource types:
Appointment
type
Extension
element type
String