Why quantity limit to 5 in invoice ?

In DocumentItem.php , it wrote like

```
class DocumentItem extends FormRequest
{
protected $quantity_size = 5;
```

I would like to know, why limit to 5 ? Base on the product , it can be over 5 characters. now, max product item limit is 99999. I also need to create two lines for creating the invoice that is that is ridiculous .