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 an existing product (little icon in the list to the right 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.