I have managed to load a child fxml(sub UI) under a parent fxml (mainMenu UI). I have created an AnchorPane with id \"mainContent\". This pane is bound to 4 sides and change
With the look at the structure of your interface I guess you better use BorderPane as your parent container with this suggestion don't use AnchorPane directly inside the BorderPane container here is my suggestion you do:
->BorderPane-Top = "your menu"
->BorderPane-Center = another container (could be SplitPane, another BorderPane etc.. but not AnchorPane I guess but you can try if you want)
->BorderPane-Bottom = another container (could be hbox with buttons or label for info or notifications etc..)