Hi Kenneth,
The buttons at the bottom of the form are implemented in an XBL component (of which there are on your form as many instances as there are buttons there). The component is implemented in process-button.xbl, and calls the server-side code from XPath with fr:run-process-by-name(…), which is implemented in FormRunnerFunctionLibrary.scala, and calls the "real" code in SimpleProcess.scala. Does this help?
If you want to read the actual code, you can find it all on GitHub. But of course, the idea is that you shouldn't need to, and that the documentation should be enough. So, you'll let us know if there is anything that you think the documentation should cover, while it doesn't.
Alex
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