All sorts of products comes with their serial numbers which uniquely define that specific item. Common use of such serial numbers is keeping track of warranty issues, product identification (ex. VIN numbers) and etc.
If You would like to add serial number to product select right side option menu within product column when adding product at the invoice.

Serial numbers window will appear.

Enter your serial numbers and confirm input by selecting Save.
Newly entered serial number becomes part of product search when searching by smart search within documents grid.
Adjust document templates to use proper variable if You would like to print serial numbers.