Consider my next trip to Japan. Considered as a plan it is a discrete object with limited detail. I do not yet even plan to take a specific flight or to fly on a specific day. Considered as a future event, lots of questions may be asked about it. For example, it may be asked whether the flight will depart on time and what precisely I will eat on the airplane. We propose characterizing the actual trip as a rich entity and the plan as a poor entity. Originally, I thought that rich events referred to the past and poor ones to the future, but this seems to be wrong. It's only that when one refers to the past one is usually referring to a rich entity, while the future entities one refers to are more often poor. However, there is no intrinsic association of this kind. It seems that planning requires reasoning about the plan (poor entity) and the event of its execution (rich entity) and their relations.
(McCarthy and Hayes 1969) defines situations as rich entities. However, the actual programs that have been written to reason in situation calculus might as well regard them as taken from a finite or countable set of discrete states.
Possible worlds are also examples of rich entities as ordinarily used in philosophy. One never prescribes a possible world but only describes classes of possible worlds.
Rich entities are open ended in that we can always introduce more properties of them into our discussion. Poor entities can often be enumerated, e.g. we can often enumerate all the events that we consider reasonably likely in a situation. The passage from considering rich entities in a given discussion to considering poor entities is a step of nonmonotonic reasoning.
It seems to me that it is important to get a good formalization of the relations between corresponding rich and poor entities. This can be regarded as formalizing the relation between the world and a formal model of some aspect of the world, e.g. between the world and a scientific theory.