Just as plants and animals have classifications, the worlds have also been classified. The categories are based primarily on their levels of technology and magic. Parallel worlds are those that have advanced in technology at a pace similar to our own. Some have societies and even geographies that are also similar, but there are others where only the increasing taxes seem the same.
Parallel worlds are further broken down into two classes: those which have developed magic along with their technology (M class), and those that have not (N class).