The custom table tab order can be ordered to put the custom tables in a specific order.
Go to Tools > Design layouts > Choose the Act entity layout to order.
In the layout designer press F4 to show the properties window (1) > select the tab control (2) > click the eclipses in the Edit Tabs property
Click the Add Tab button (1) add the name of the custom table and position the tab using the Move Up and Move Down buttons (2). When the layout is loaded Impact will put the custom table where you placed it with the Edit Tabs window.