Hi Nico
Some background. When Adobe first launched LiveCycle, the first capabilities they needed to expose was encapsulated as Forms Server, and it provided:
1. Rendering a form. This would take a PDF or XDP file, plus some xml data, merge them together, and produce a PDF file.
2. Handling a form submission. This would take the data from an http request coming from a Submit button on a form, process it, and extract the XML data.
The Forms "Submit" QPAC is basically the submission API encapsulated as a QPAC. If you want to know exactly what it does, the best place to look is the LiveCycle Forms documentation.
So, to your question:
If a document variable contains a PDF form, you can use the Form Submit QPAC to extract the XML data.
A form variable is really just the XML data for a form, plus a reference to the PDF or XDP file that should be used for displaying the data - so if you assign the extracted XML to the appropriate node in the form variable, it should work.
I hope this long ramble is helpful.
Howard
http://www.avoka.com