Hi,

Firstly when you confirm sale order it will create Draft Delivery Order and its line. Now, line object would be stock.move, after that when you perform any kind of operation on that DO, you will see object changed. It is now stock.pack.operation.

It is coming from this method:

do_prepare_partial() of class stock_picking

you will find this line.: pack_operation_obj.create(cr, uid, vals, context=context) 

It is creating pack operation from move, and where pack_operation_obj = self.pool.get('stock.pack.operation')

Now if you want to stop this You have to customise the code, override method do_prapare_partial or try by commenting line where pack operation is created. 

Thanks,

Chandni