x1 chooses/selects option x2 (choice) at the exclusion of all others among the collection of all options x3 (complete list or set).
x3 includes x2 when fully specified. Any element/entry/term of x3 which is not an x2 is not chosen. If x2 is a singleton, then x3 forms a radio button selection or the options in a stereotypical multiple choice test. See also: cuxnajo'e.