Depending on what exactly you are aiming to do, you may want to consider using the Secondary Unit modules and creating a new Unit of Measure Category for cartons, and a new Unit of Measure in this category for just cartons (with rate / factor of 1).
Here is some more info: Uom Conversion Inter category on product
Related modules:
- Account Invoice Secondary Unit Module — trytond_account_invoice_secondary_unit latest documentation
- Purchase Secondary Unit Module — trytond_purchase_secondary_unit latest documentation
- Sale Secondary Unit Module — trytond_sale_secondary_unit latest documentation
- Stock Secondary Unit Module — trytond_stock_secondary_unit latest documentation