There are two kinds of variables: variables refer your screen (e.g. the x and y values of where it was last touched) and object variables refer a specific character, shape, image, or text (e.g. the x and y value of Chillana). You got that, though.
Each object variable can apply to a single, specific object or to "Self". When you add an object variable, it's automatically set to Self. You can change it to another object if you want.