This case-sensitive code system http://hl7.org/fhir/inheritance-control-code defines the following codes:
Code
Display
Definition
Additive
Additive
If listed in the differential would 'add to' what's in the base snapshot and propagate what's in the base as well
Fixed
Fixed
if listed in the differential, would 'override' what's in the snapshot (but must be a subset or same as base snapshot). Propagate what's in the base only if extension doesn't appear in the differential (and must check differential against base)
Overrides
Overrides
if listed in the differential, would 'override' what's in the snapshot (but can be different than base snapshot). Propagate what's in the base only if the extension doesn't appear in the differential (no need to check differential against base)
frozen
Frozen
Can only be listed in differential if identical to base and is propagated if not in the differential. Propagate what's in the base always. Ignore what's in the differential - though you can yell if it's different
no-copy
Does not propagate
Does not propagate from base and value in base has no direct impact on what value can be in differential of derived element (though there could be methodology rules - e.g. FMM)