The semantics of the fact that a db-value is missing are NOT the same as the semantics of the db-value itself. The former fact applies to any db-value, no matter what its type. The latter fact has semantics depending heavily on the domain (or application data type) from which the attribute draws its values.
Like a variable, a mark is a place-holder. However, it does not conform to the other accepted property of a variable: namely that semantically distinct missing values are represented by distinctly named variables.
SIGMOD RECORD, Vol. 15, No. 4, December 1986
This I think draws a clear line between the aspect of missing information that I’m interested in, and the aspect that I’m not interested in.
If the missing information is interpreted from the context of the “db-value” semantics, then I’m not interested. What I am interested in is the semantics of the missing information and the “domain (or application data type) from which the attribute draws its values.”