Quote from "Must Support" section:

In situations where information on a particular data element is missing and the US Core Responder knows the precise reason for the absence of data, US Core Responders SHALL send the reason for the missing information using values (such as nullFlavors) from the value set where they exist or using the dataAbsentReason extension.

Quote from "Missing Data" section:

If the source system does not have data for a required data element (in other words, where the minimum cardinality is > 0), the core specification provides guidance which is summarized below:

For non-coded data elements, use the DataAbsentReason Extension in the data type
Use the code unknown - The value is expected to exist but is not known.

There is a kind of conflict on what DAR code to be included.
Question 1: if the element is optional + MS, shall it use the any DAR code or unknown only?
Question 2: if the element is required, shall it use unknown only?
Question 3: if the element is required + MS, shall it use unknown only? (the second quote says "required" not "required and Must Support")