Determines the behavior of a widget with respect to an operation.
$op: The name of the operation. Currently supported: 'default value', 'multiple values'.
$instance: The field instance array.
One of these values:
function field_behaviors_widget($op, $instance) {
$info = field_info_widget_types($instance['widget']['type']);
return isset($info[$op]) ? $info[$op] : FIELD_BEHAVIOR_DEFAULT;
}