my question is more related to the field names in the database so that I could create a custom theme that would show the customer’s delivery address on both a ProForma Invoice & a PO
I don’t think that are fields not exposed in the standard theme.
First, all field names are those used in the Plain theme built into the program. If you copy it to a new custom theme, you will have them all. Understand that many of the variables are arrays.
Second, for pro forma invoices, use a sales quote and give it a new title. See Create sales quotes | Manager (and keep reading all the way to the end).
Third, you don’t need a custom theme to include a delivery address. You can add one right below the billing address in the sales quote. Put it into the same field. If you want the delivery address elsewhere on the sales quote, you can add a custom field. Only if you don’t like the resulting position do you need custom theme.
Fourth, purchase orders are unrelated to customers. Purchase orders go to suppliers. Nevertheless, to include a delivery address on a purchase order (for yourself), use a custom field.