They could be your areas of responsibility, if that makes sense in your particular system. It’s just that the behavior of areas in Everdo makes them more useful for some things and less for the others.
For example, I most often would like to combine several Areas of Responsibility into a single Next Actions list. This doesn’t work if each Everdo area corresponds to just one Area of Responsibility. This is only my particular approach, but I treat the list of Areas of Focus merely as a review tool and it has no bearing on the “tactical” part of my GTD workflow.