By denotatum (probably this is the right term here, not denotation) I mean the situation those tenses describe. In semantics it's important to make distinction between the following three components that constitute the structure of the meaning: denotatum, concept and pragmatic factors. The denotatum component is a picture of "a person on the chair". This picture can be viewed upon in different ways. This is the concept level where you interpret the situation through the system of meanings forming the semantics of this or that tense category. How it is done I described in my post above.