Steve Mellor and I independently came up with a characterization of the three modes in which people use the UML: sketch, blueprint, and programming language. By far the most common of the three, at least to my biased eye, is UML as sketch. In this usage, developers use the UML to help communicate some aspects of a system. As with blueprints, you can use sketches in a forward-engineering or reverse-engineering direction. Forward engineering draws a UML diagram before you write code, while reverse engineering builds a UML diagram from existing code in order to help understand it.