1. Turn on class tracking. Go to Settings (gear icon), then Account and Settings, then the Advanced tab. Under Categories, switch on Track classes. This option doesn't exist in the menus until it's enabled.
2. Decide what a class should represent, before you start adding them. Classes typically map to departments, locations, business lines, or distinct revenue streams. The structure matters more than the number of classes; a handful of clear categories reports better than a dozen overlapping ones.
3. Go to Settings, then All Lists, then Classes. This is the dedicated list for every class in the company.
4. Click New. Name the class, and if it's a sub-category of a broader one, check Is sub-class and choose the parent.
5. Save, and repeat for each class you need. There's no hard limit on how many classes a company can have, though QuickBooks works better with a clean, deliberate structure than a long sprawling list.
6. Assign a class on each transaction going forward. Invoices, expenses, and most other transaction types get a Class field once tracking is turned on; assign it the same way you'd assign a category or customer.
7. Turn on the warning for unclassified transactions, if you want a safety net. Under the same Advanced settings, Warn me when a transaction isn't assigned a class flags anything that slips through without one.