Comment by adzm

This reminds me of the sixteen or so different flavors of null used in HL7! https://terminology.hl7.org/en/CodeSystem-v3-NullFlavor.html though in practice I've only seen a handful of these. Notably:

NI No Information - the default, it is unknown and why it is unknown is also unknown

UNK Unknown - the most common other than NI

ASKU Asked but Unknown

NASK Not Asked - we don't know because we didn't ask

MSK Masked - hidden due to privacy or legal etc

NA Not Applicable

NAV Not Available - for example patient might be unconscious or doctor might be unreachable, temporary