Hi andrii-rudchuk-t!
Yes, you're right that the One to entire transaction option is only applicable to sales transactions and not to purchases or vendor-related transactions. If you wish to suggest a change to this, you can send feedback to our product development team for consideration in future updates.
Class tracking is a feature that allows you to categorize transactions by class. This can help you track income and expenses in different departments, locations, and other criteria. You can learn more about it in this article: Get started with class tracking in QuickBooks Online.
If you have additional questions about the said feature in QuickBooks Online, don't hesitate to reach out for help. We are here to assist you.