Hello there, Chai Wanma. 

Yes, there are a few limitations on when and how we can use classes in QuickBooks. The feature is available for Plus subscribers only in which we only add up to 40 classes and location combined. We can also assign classes either One to each row in transaction or One to entire transaction. 

Here's how we can turn on and create classes if you wish to upgrade and use the feature:

1. Go to the Gear icon and click the Advanced menu.
2. Select the Categories section.
3. Check the Track classes box.
4. If you wan to receive prompts in case we're unable to enter a class click the Warn me when a transaction isn’t assigned a class box. 
5. Select either One to each row in transactions or One to entire transaction. 
6. Click Save and then Done. 

Let me show you how to create your first class: 

1. From the Gear icon again, click All Lists. 
2. Click Classes. 
3. At the upper-right, click New. 
4. Enter a Name and click Save. 

Here are a few articles that will give you more details about using the class tracking feature:

• Create and manage classes in QuickBooks Online
• Track your transactions by class

You can always reach out to me if you need more help. Thanks!