Thanks for coming back with more detailed information on your concern, @cpa_chang. 

Yes, you're right. You can assign a class at the transaction level. See the following transactions below:

• Invoice
• Sales receipt
• Estimate
• Sales order
• Statement Charges
• Refunds and credits
• Check
• Credit card charges
• Bill
• Purchase order
• Paycheck

Since class tracking can be used in most transactions, you can generate reports to compare the balances across multiple classes, departments, or locations by filtering, sorting, or totaling reports by class in QBDT.

You also want to check this link to learn more about setting up and using class tracking in QuickBooks Desktop. 

Feel free to message again if you need more QuickBooks help. Take care!