We use Approvalmax synchronizing data with QBO, but we cannot select tax codes at AMAX side suddenly, whereas all other codes such as GL account / Class are synched.

Hello there, KENGO. I'm chiming in to share some details about how syncing transactions works in QuickBooks Online (QBO).

We know how important it is to get your data in sync in our program. However, QuickBooks depends on the information provided by your third-party application. We suggest contacting the third-party app's support for this concern. They can help you check your setup and pinpoint the root cause. 

I've added these links below about importing and exporting data in QuickBooks:

• Import data from other software into QuickBooks Online
• Common questions about importing data to QuickBooks Online
• Export reports, lists, and other data from QuickBooks Online

You can return here anytime you need assistance managing your data in QBO. I'm always around here to help.