Guess, nobody suggest it yet, at least by my searching queries. The point is to allow users to choose values from context-dependent tree of options. Example: Choosing our location. We're choosing country, then in the next menu we're choosing region (county or state), then we're choosing town, ect. The final custom field value may be just as simple text. May be helpful when you're using a lot of options for custom filds and want to standardize the appearance. Thanks for looking.