Hi,
The question is a little bit too large to have an absolute answer.
There is no link between the serial and the product. There is just a link between the product and the pack.
You can also have a link between the serial and the user but that's a different story.
Now, if you take the configuration where you have a pack which is just assign with one product ; by configuration all serials of that pack will be assign to that product.
The link between the product and the serial is based on the [m:n] relationship.
product <-> pack <-> serial
en.wikipedia.org/wiki/Many-to-many_(data_model)
But even if you have that structure, you can still use it has a one-to-one or one-to-many ; the data will do it.
At the end, HikaSerial provide the most flexible relationship possible ; it's up to you to restrict it to do what you want to do.
So your question is a database relationship question more than an HikaSerial one.
Regards,