It seems like this topic could be greatly improved, when time permits. I bet it's a lot more complicated than this overview suggests. Most of the content was deleted in the move from INVOICES Help to DOCUMENTS Help. Info on form appendices was deleted, as well - you're supposed to use multiple doc specs, instead. Probably the same is true here. Emailed Carl 4/2011 - no response yet. /EP
Some invoices include one or more pages of additional information such as work descriptions, order descriptions, and so on. These additional pages that follow the actual invoice are referred to as appendices. Appendices always have a different appearance than invoice pages.
The system does not extract information that appears on the appendices. However, you can save the images of the appendices – see Page logic: Overview and The dialog: settings.