The Invoicing plan tab is shown if you have selected the setting Manage Invoicing Plans on Quotes/Customer Orders, under Invoicing plans under the Sales tab in the Settings procedure. This tab is greyed out if no invoicing plan is registered on the customer order.
Here you will find information about the invoicing plan provided that an invoicing plan is registered on the order. Read more about the Invoicing plan tab in the Register Customer Order. No information can be updated under this tab.