This case-sensitive code system http://hl7.org/fhir/us/cdmh/CodeSystem/pcornet-death-sources defines the following codes:

Code	Display	Definition
L	Other, locally defined	Other, locally defined may be used to indicate presence of deaths reported from EHR systems, such as in-patient hospital deaths or dead on arrival.
N	National Death Index	Death Source is National Death Index.
D	Social Security	Death Source is Social Security.
S	State Death files	Death Source is State Death files.
T	Tumor data	Death Source is Tumor data.
DR	Derived	Death Source is Derived.
NI	No information	Death Source has No information.
UN	Unknown	Death Source is Unknown.
OT	Other	Death Source is Other.