x1 (value, state, form) is the asymptote/limit [Calculus sense] of function/dependent variable/limitand x2 as independent variable(s) x3 tend(s) toward point/respective value(s)/tuple x4 along path/from direction x5.
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.