When you submit the form, the value is presented with debug_dump(), so strings are quoted and array values listed.

:
: - Stop the submitted form from bring processed
: - Don't process the form, redirect to another page (e.g. login after session timeout), and remember the values for when they return.

or back to examples 

<?php
	$field = new form_field_postcode($form, 'Postcode');
	$field->format_error_set('Your postcode does not appear to be correct.');
	$field->required_error_set('Your postcode is required.');
?>