Exactly the same struggle I’m having @MTNathan. Through some experimentation I’ve discovered that there is a predictable order to how NetSuite renders the tabs. Custom Child Records are rendered first, followed by any Registered Sublists and finally any Registered Subtabs that are linked to the form and have at least 1 visible field contained within it. It looks like among the Custom Child Records and Sublists of tabs they are rendered alphabetically but I can tell if by Record Name, Label or Record ID. The Registered Subtabs however are appended to the end and follow the layout order defined on the Custom Form Record.