You haven't identified what you want these boxes to provide. They will Fill from the database for the Customer Record, since you are asking about Header fields, excluding a few that are Freefom text, such as Customer's PO # is a text box. Here's how to learn:
When you activate a field (not really a Box) and then look at it using Create Invoice, if you see a little dropdown arrow, that means it will come from a List; example: Terms. If you see a little square calendar, that will be a Date field; example: Ship Date. Otherwise, it is Text.
Next, the "Custom SO Template" = From a Sales Order (SO) as a Fulfillment process. The programs that support Back Order Tracking allow you to have the columns for tracking the Qty ordered vs delivered, for each invoice created that is fulfilling one or more SO for that customer name.
Now, the Differences you see in Rock Castle Construction Header can be learned by Edit Customer; click on Additional Info, and on the right you see some Custom Fields, and the Define Fields button. You can use custom fields for Names and for items. Once you enable them, and you Label them, you can Select them for use in templates.
Use that scrollbar to see that the Custom FIelds of Spouse and Contract are why you see More Fields. And there is a Limit to how many Custom Fields you can enable and use. I cannot specifically recall; I think it is 14 fields, but only 5 max for any Name Type.
Let's also examine that using Provided fields is better than lots of Custom Fields. For instance, "Contract" for Rock Castle Construction was Before the customer center now offers Account #. The reason to avoid custom fields, when there are fields available that you can use for that same purpose, is that the provided fields are already part of the relational database and report better. I use Account Number, not Custom, for Contract #, because it shows in more places. Also, excluding Enterprise, you cannot define a custom field as "from a list" or "as a date" which means there is no Consistent entry that makes report Filters work well. Example from when I teach this lesson: "B-Day" in Rock Castle Construction (Birthday) can be entered as "5 apples" because there is no Date Format checking and there is no little calendar icon.
So, again, without knowing what you want to use for the data in these Header fields, it is hard to point out the better function. Example: Ship Date can be Relabeled on the Template to be "Class date:" for a Tutoring or Training business, because "date of delivery" is the date they Deliver the materials to the students by holding a class session.
Hope that helps.
Please see my attachments.