If you want every transaction from a specific store to sync into QuickBooks or Xero with a set class, you can turn on the Default Class option in that store's settings. Once enabled, all data from the store will sync with the class you choose.
Steps
Go to Stores and Files in the left navigation menu.
Click on the store you want to update.
Open the QuickBooks Settings tab for that store.
Scroll down and click to reveal Advanced Settings.
Check the box labeled Use Default Class.
In the Default Class Name field that appears, type the exact class name as it exists in QuickBooks.
Once saved, every transaction synced from this store will be tagged with the class you entered. If the field is left blank or the box is unchecked, transactions will sync without a class.