x1 is a general metropolitan area with (population, commercial, industrial, business, economic, infrastructural, vel sim.) center/focal point/downtown/main or dominating urban area x2, with supporting semi-urban/inner suburban region x3, more (outer) suburban region x4, and exurban region x5.
The divisions are fuzzy and none of the specifications are inherently legal/government-recognized. Generally, x2 is primarily oriented toward work/daily activity and does not have any or much support for actually inhabiting such a place/region (except penthouses), it is the downtown of a relatively big city; x3 begins to support habitation by the general public but is still urban/developed within an urban model (it may include boroughs); x4 is almost exclusively habitable areas with local or smaller business (sometimes, there will be fractalic hubs with their own fairly well-developed commercial/business centers, but these will tend to be smaller and - as stated - more focused on commercial or smaller business interests (or a commercial chains or malls), rather than economic or industrial pursuits; they will also have a notably smaller servicing or permanent population and population density; but it is all fuzzy and there will be plenty of exceptions); x5 is the outer region of a metropolitan area and it will be somewhat more sparsely populated and will grade toward a rural lifestyle or simply human-uninhabited/natural territory toward its outer edges, even though such features/lifestyle will not be ubiquitous. Megalopolises are bigger than x1, typically (but not necessarily); x1 contains (in a general way) or 'owns' or is the orientation of all of the other xis; it the x2 area/system. Outposts/frontier settlements do not belong to this structure. See also: tcadu, jarbu (which this word generalizes).