Good morning, @b0bb. 

Thanks for posting here in the Community. 

At this time, QuickBooks Online doesn't have the option to tie a customer to a sub-account. I'm going to submit feedback to our Product Development Team to review and consider making this an option soon. 

You can check out this detailed article as it has further information about user permissions: User types and user permissions in QuickBooks Online. 

For now, QuickBooks Online Advanced has a lot of different customization features like banking, sales, payroll, and more. You can refer to this article: Add and manage custom roles in QuickBooks Online Advanced. 

If you have any more questions, I'm always here to help. Have a safe and productive rest of your day!