Extension:Associated Encounter

I am learning about Encounter from FHIR website.What are Extension:Associated Encounter.Can some one please describe it to me with simple example.I am just beginner to FHIR community?

The associated encounter extension can be found here: http://hl7.org/fhir/extension-encounter-associatedencounter.html

You can read about how extensions work in general and see examples of what they look like here: http://hl7.org/fhir/extensibility.html