SubItem::setParent

public function setParent($instance)
{
	if($instance !== null)
	{
		if(!is_a($instance, '\Bitrix\Tasks\Item'))
		{
			throw new ArgumentException('Illegal parent instance passed');
		}
	}

	$this->parent = $instance;
	$this->setParentId($instance->getId()); // 0 or effective ID
}