This case-sensitive code system http://hl7.org/fhir/linkage-type defines the following codes:
Code
Display
Definition
source
Source of Truth
The resource represents the "source of truth" (from the perspective of this Linkage resource) for the underlying event/condition/etc.
alternate
Alternate Record
The resource represents an alternative view of the underlying event/condition/etc. The resource may still be actively maintained, even though it is not considered to be the source of truth.
historical
Historical/Obsolete Record
The resource represents an obsolete record of the underlying event/condition/etc. It is not expected to be actively maintained.