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

Selection:
: - 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
	$items = array(
		1 => 'Item A',
		2 => 'Item B',
		3 => 'Item C',
	);
	$field = new form_field_checkboxes($form, 'Selection');
	$field->options_set($items);
	// $field->options_disabled_set(array(2 => true));
	// $field->options_info_set(array(2 => ' - Disabled'));
	// $field->required_error_set('Your selection is required.');
	// $value = $this->values_get();
	// $value = $this->value_keys_get();
?>