The rule implemented by the function f(x) normally applies to words related to locations or countries, converting them into decimal values. However, it does not seem to hold for all inputs. For example, inputs like "Marco Rubio", "Zircon", "The Barber of Seville", "pencil", and "Bear" do not follow the rule and return undefined.

Keep in mind that this is an initial analysis based on a limited set of inputs. It would be helpful to run more tests and observe additional outputs to get a more comprehensive understanding of the rule implemented by the function f(x).