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)