Every node within a TreeGrid can be selected. But sometimes you only need the leafs or only a few of them. It is possible to filter the selection later, just before you execute an action on the selected nodes. But the better way is to prevent a selection on obsolete nodes. Use a custom SelectionModel for that.

public class CustomSelectionModel extends GridSelectionModel<ModelData> {

	public CustomSelectionModel() {
		addListener(Events.BeforeSelect, new NoSelectionListener());
	}

	private class NoSelectionListener
		implements Listener<SelectionEvent<ModelData>> {

		@Override
		public void handleEvent(SelectionEvent<ModelData> se) {
			ModelData model = se.getModel();
			if (model instanceof Something) {
				se.setCancelled(true);
			}
		}

	}
}

You can also use properties of the model to make a decision, but sometimes you can simply ask for the instance type (i.e. folders and files).