What you're describing is called a "hierarchical drop-down" or "hierarchical menu" or "hierarchical value list". If you do a search for those terms, you should find at least a few threads discussing various techniques for creating such a beast. (Try https://fmdev.filemaker.com/message/67218 for a start.) It's definitely possible, and different people have their favorite methods for implementing. Some involve using global fields with "utility" relationships from the first layer to the second layer and so forth. Others are more sophisticated, but might be a little harder to wrap your brain around.