src/Form/Hydrator/Strategy/TreeSelectStrategy.php
public function allowSelectMultipleItems()
    {
        $flagOrCallback = $this->allowSelectMultipleItems;

        return is_callable($flagOrCallback) ? (bool) $flagOrCallback() : (bool) $flagOrCallback;
    }
public function allowSelectMultipleItems()
    {
        $flagOrCallback = $this->allowSelectMultipleItems;

        return is_callable($flagOrCallback) ? (bool) $flagOrCallback() : (bool) $flagOrCallback;
    }
Are multiple selections allowed?

@return bool