OneSaas supports two different layout types for Transactions (Invoices or Orders): Service Layout and Item/Product Layout
Each line of the transactions has a description, an allocated account from your Chart of Accounts a TaxCode and a Total for the line:

Products or Services imported as Service Layout don’t require a ProductCode or SKU.
In Item/Product Layout every line represents an existing Item or Product that was already pre-defined by your accounting, CRM or eCommerce system. Each Product/Item requires a unique ProductCode/SKU that is used to uniquely identify the product. If you try to import transactions/invoices in OneSaas and an item on the imported order/transaction is missing a ProductCode then OneSaas will fail to import the transaction.

Products or Services imported as Item/Product layout require a valid ProductCode or SKU.