The rule applied by the function seems to hold for geographical locations, as we observe that inputs like "Spain" and "Mexico" yield numerical outputs (172.79 and 1.74, respectively). However, for other inputs like names of people ("Marco Rubio") or objects ("Zircon", "pencil") and even cultural references ("The Barber of Seville", "Chichen Itza"), the function returns undefined. Therefore, it appears that the rule implemented by the function f(x) only applies to a specific subset of the domain, which consists of geographical locations.