Static Inclusion
fx:include
can be placed in the parent FXML file to statically include a child FXML with it's own Controller.
For example (from the Oracle FXML Introduction), given the following markup:
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<VBox xmlns:fx="http://javafx.com/fxml">
<children>
<fx:include source="my_button.fxml"/>
</children>
</VBox>
If my_button.fxml contains the following:
<?import javafx.scene.control.*?>
<Button text="My Button"/>
the resulting scene graph would contain a VBox as a root object with a single Button as a child node.
Dynamic Loading
Loading new fxml in the same scene describes how to load the new FXML files dynamically into a replaceable child Pane.