This option determines what value the field will return when the submitted

value is empty. It does not set an initial value if none is provided when

the form is rendered in a view (see `Data` or `Placeholder` options).

the form is rendered in a view (see ``data`` or ``placeholder`` options).

It helps you handling form submission and you can customize this to your needs.

For example, if you want the ``gender`` choice field to be explicitly set to ``null``