I know it's late, but I thought I'd provide some info. In the UI, look at how some "fields" are really from a List; you are defining something that gets saved on a list and can be selected later. Not everything is freeform text. Next, some fields that are text are line info and some are transaction info. That's why columns such as Other, as Line info. The Transaction Header is where you found FOB, because Freight On Board applies to the entire order.
Service Date is going to be the source date of a billable transaction (such as timesheet activity, bills, check purchases of items) and seems to have been provided for Statement Charges, as opposed to invoices. That makes the date of sale for Statement Charges, which is AR bypassing the use of an Invoice and bypass all "calculating items" and functions, such as Sales Tax. That's why we can use them in MT = we don't have sales taxes. Statement Charges work great for things like landlord rental activity, because they want the charge, they want a due date, and they won't print or email anything to anyone. Just as making an invoice Pending was replaced with Estimates decades back, a lot of the functions were replaced with newer functions, but old functions were left for backwards compatibility and that would help explain statement charges.
There are almost no changes to .iif, because it is deprecated and unsupported, from around the time the company data file was converted from flat files to relational. That also means the other improvements are not incorporated. That includes Contacts. The Alt Contact (seen in the Cust .iif reference as CONT2) has all been completely translated into the Contacts function that now is a tab in the Customer/Vendor Centers, with maintenance of multiple contacts now possible. But sometimes, you see these fields when you want to Add Fields to a Template, such as adding Contacts to a statement or invoice template, will reveal old code.
For the email, it helps to notice how you can now select from a rotating set of labels for info related to customer or vendor names, but only the field that holds the email address and is the email field is a function (not just for display notation, but makes something happen). Then there are things you learn only by testing, such as Other Names have a "print as" but it's not a field; it's from the first billing line.
"I am simply looking for a replacement field for FOB with a longer character length that is an allowed field on the !TRANS line of the IIF file."
You can use a line entry treating it as Descriptive only (no item, qty or amount). You see this for Change Orders. It's not a great method, but it works.
