To structure your FXML better, you can use fx:define to separate usage and declaration of various elements. For example, the preceding examples with includes can be rewritten by defining the include first and using it later through the $ prefix. Refer to the following code snippet:

<?import javafx.scene.layout.*?>
<?import javafx.scene.control.*?>
<HBox xmlns:fx="http://javafx.com/fxml" fx:controller="chapter4.includes.FirstController">
    <fx:define>
        <fx:include fx:id="myLabel" source="MyLabel.fxml"/>
    </fx:define>
    <children>
       <StackPane children="$myLabel">
    </children>
</HBox>

Another common use case for define is setting a toggle group for radio buttons:

<?import javafx.scene.layout.*?>
<?import javafx.scene.control.*?>
<HBox xmlns:fx="http://javafx.com/fxml" >
    <fx:define>
        <ToggleGroup fx:id="toggleGroup"/>
    </fx:define>
    <children>
        <RadioButton text="radio1" toggleGroup="$toggleGroup"/>
        <RadioButton text="radio2" toggleGroup="$toggleGroup"/>
        <RadioButton text="radio3" toggleGroup="$toggleGroup"/>
    </children>
</HBox>