Thanks for providing a screenshot about your concern, @lucie7. I'm here to provide some details about adding class to the transactions in the Gain or Loss on exchange rate account.

Currently, adding class to an entry under the Gain or Loss on exchange rate account is unavailable. In QuickBooks Online (QBO), the exchange gain and loss amounts are calculated separately from the source transaction once the payment is applied. That said, you can only assign a class to the invoice and not at the time of payment.

As a workaround, you'll want to move the exchange gain or loss transaction to your desired class through a journal entry. Before doing this, I highly recommend asking for guidance from your accountant to ensure you'll assign them correctly and avoid messing up your books.

For more information on managing classes in QBO, feel free to check this article: Get started with class tracking in QuickBooks Online. 

Need to have additional guides in performing your QBO tasks? Feel free to browse this page to find one: QBO Help articles page. This includes a compilation of help articles that you can access or bookmark on your browser. 

Please know that I'm always here ready to assist if you have further concerns about adding classes in QBO. I'd also appreciate it if you can add updates about the recommendations. Have a good day ahead and take care.