This widget is the classical FIELDSET HTML tag.

An empty fieldset:

A fieldset with something inside:

$this->build("p4a_fieldset", "fieldset");
$this->fieldset->setWidth(300);
$this->fieldset->setTitle("This is a fieldset");
 
// if you want to view some objects inside the
//fieldset, you need to anchor them with:
$this->fieldset->anchor($this->field);