A variable can be reused across your document, but it will always have the same expression. If you want different expressions, you will need to use different variables. Variables are different if their names are different. You specify the name of a variable between the two @ delimiters in the content of your text box.

Suppose you have two text boxes, with the content of both boxes set to "@zip@". Double-click one of your text boxes and replace "@zip@" by "@onecode_zip@". When you click outside the box you will have two variables: "zip" (defined in the first text box) and "onecode_zip" (defined in the second text box), each with its own expression.