Hello there, @RHE55.
I'm here to help share some insights about customizing checks in QuickBooks Desktop.
At this time, being able to add a comment tab for checks in QuickBooks Desktop isn't an option for us. However, you can use the Memo field on your check to enter any necessary information that you'd like to extend into your clients.
The ability to customize forms is only available for sales transactions. To give you more information about modifying checks and customizing forms, please refer to these resources:
On the other hand, our developers are constantly working to improve our product to help you manage your business easier. I'd encourage you to visit the What's New section on your QuickBooks Desktop regularly, to be updated about our latest news, features, and updates about what our Product Care Team is working on.
Please let me know if you have any questions about this or anything QuickBooks related. I'd be happy to help. Have a wonderful day!