You can have products that has the same product number, but with a unique serial number. I.e. multiple of the same bike, that only differs on serial numbers.
You can create products with serial numbers in three ways:
Add a new product and under "stock" select "Bike with Serial number."
Edit an existing product and change the "stock" to "Bike with Serial number."
Duplicate a existing product (little icon in the list to the left of the product). And add a serialnumber to the duplicated product.
When you add a serial number to a product you have to delete the barcode, the system will automatically use the serial number as barcode going forward.