You may have used the JavaFX Scene Builder tool to create the user interface for a JavaFX application. The underlying data format for the UI that you create with Scene Builder is an XML dialect named FXML. Here's an article that I wrote for Oracle Technology Network to help you understand FXML.
By the way, the REST communcation in the example program uses REST/FX. As a heads-up, DataFX is becoming a de-facto standard that you might check out as well for data communication from a JavaFX application.