It will be the default $id_parent for options requested by code
X::dump($opt->getDefault());
// (int) 0
// Default root option
$new = $opt->fromCode('test');
// false
// Option not found
$opt->setDefault($new);
// Default is now 5
X::dump($opt->getDefault());
// (int) 5
X::dump($opt->fromCode('test));
// (int) 24
// Returns the ID (24) of a child of option 5 with code 'test'
$opt->setDefault();
// Default is back to root
X::dump($opt->getDefault());
// (int) 0
function($uid)
{
if ($this->check() && $this->exists($uid)) {
$this->default = $uid;
}
return $this;
}