Each of those contains a value with different semantics.
The key word in your sentence is "whatever". It is exactly
that lack of
precision that will prevent reaching common, useful understanding and
consensus.
And I am saying that it does not matter in the slightest. That
discussion of the semantics is unnecessary and counter-productive.
Whatever the semantics of the headers are is already decided.
Therefore there is absolutely nothing this group can do to change
them.
Far from it being essential to have a rigid formal model of the
morass that is SMTP deployment it is essential to design a spec
that is entirely indifferent to it.
Everything the receiver does is non-normative.
Phill