This page describes how local extensions are implemented.
Local extensions were created to have base code that will be used for real extensions when they will be registered by OpenMRS community and ready to use.
In ExtensionsURL class we keep URLs of extensions that we are using. Because some extensions are not registered yet we decided to use simple name of extensions as replacement for URL.
Above code snippet shows few local extensions that we created.
When some extension will be registered we should use it in FHIR code. Let's suppose that "changedBy" extension is registered under URL - "https://openmrs.org/fhir/StructureDefinition/changed-by". All you have to do is replacing value of CHANGED_BY_URL with this URL.