Hi there, Marianne18.

Thank you for visiting the QuickBooks Community. I'll be sharing details on how Classes are assigned in QuickBooks and how it works.

Class tracking lets you track account balances by department, business office or location, separate properties, or any other meaningful breakdown of your business. However, as previously stated, only the QuickBooks Enterprise version has the option to assign classes to your expense and income accounts, as well as the option to select Assigned classes to Accounts in the Preferences.

Since you're using the QuickBooks Desktop Pro, you might want to assign classes to some of your transactions rather than your accounts. Then, you may follow the workarounds provided by @ReyJohn_D above as a workaround so you're able to choose the right income or expense account when creating transactions like invoices, check, etc. This way, you can connect the correct class to its corresponding account accurately. 

For additional information on how the Classes feature works in QuickBooks, you can open this article: Set up and use class tracking in QuickBooks Desktop.

Lastly, you can click this article to see details on how you can run and customize reports on QuickBooks Desktop so you'll be able to track your entries associated with classes: Filter, sort, or total reports by Class.

Tag me in your reply if you have other concerns or questions about the Class feature. I'll be around to back you up. Keep safe!