I have one big one for "reality". It covers everything from laws of physics to the way the legal system works and what is right and fair, and so on. It is what I use day-to-day to live my life and understand and predict the world around me.
In fact, when that compartment has its foundations attacked it causes a lot of stress for me - an ADR case for the company some time back broke my view of right and justice and the way things work and caused me a lot of stress as a result.
But I have many other compartments, and they are generally built on top of that core "reality" view of the world, but with variations.
I have one called "Stargate" for example - I can watch an episode of Stargate and for that time I am in a world where the laws of physicals and the morals and rules of the universe are based on "reality" but with the slight tweak that there are stargates creating worm holes to other realities and a group of people that travel to other worlds. I can even assimilate new knowledge and update the compartment with that new knowledge whilst watching the TV show.
I have another called "Star trek" (there may be a theme here), and so on. In each of these I can assess and predict outcomes and add to the base knowledge of "how things work" quite independently to other compartments.
None of these encroach on "reality". I wonder if there are people for which that is not so easy?
In dreams, I can put myself in one of these and sort of live a dream life within a variation of reality. I can also have dreams that are very much locked to my "reality" compartment and have had dreams where I solve issues on software design that I can work on when awake later!
I wondered why we have these. Why do we have this ability to create compartments in our mind?
If we did not have a way to separate such things from reality; if we had only one compartment, pretty much all literature and film and fantasy would be impossible. It would not make any sense to us.
I pondered one theory of my own - a key thing that we as people have is the ability to model another minds. That is - to imagine what someone else is seeing, and that be separate to what we know and see. This is key to the idea of deception, and a massive evolutionary advantage for us to do. Unlike very young children we do not assume all other people know the same things we do - we create a mental model of what they know. This allows us to predict what they will do, and so we have an advantage. It is probably a key reason we have such big brains!
This means we have to have a concept of compartmentalised models of other people in our heads - the more the better. It is hierarchical in that we assume they all have a view based on "reality", but we can model each person and what they know and what they have seen independently.
I suppose that people with multiple personalities perhaps have this mechanism slightly broken - forgetting or changing which is the "base" model.
This system allows fantasy as it allows us to model non real people and what they know and see and how they would act. I think fantasy is a side effect of basic deception and modelling of other's thoughts.
In some ways that is an evolutionary back-fire. We have no real "need" for fantasy and it is using valuable mental resources that we should have used to model what a lion has seen in the jungle...
The good news is that modelling others is perhaps one of the main ways we "live on", in the minds of others, when we are gone. People with which we interact make their models of us all of the time and those models live on for a while, predicting what we would say and do.
So this process creates both fantasy and ghosts...