There are some people who still aren't completely clear on the expectations around 'required' bindings.  Thus suggest adding a paragraph to the 'required' section:

"The use of an extension (e.g. [data-absent-reason]) cannot be used to bypass required binding requirements.  The binding must be satisfied regardless of what extensions are present."

and to the 'extensible' section:

"The use of an extension (e.g. [data-absent-reason]) cannot be used to bypass extensible binding requirements.  For example, if a value set includes the concept of "unknown", then a [data-absent-reason] of "unknown" cannot be sent without sending a Coding with the "unknown" concept.  (In which case also sending an 'unknown' DAR would be inappropriate and unnecessary.)