You would need a product like OGL (Overlay Generation Language) to create overlays and PPFA (Page Printer Formatting Aid) to create formdefs and pagedefs.
Electronic overlays can be used to mimic a pre printed form or statement, add logos/signatures, etc. A formdef controls the logical presentation of the document, and other things like duplex/simplex, what overlays to use, etc.
A pagedef controls how the data is placed on th physical page.