[Scribus] scribus xml versus xsl fo?

[Mark D. Anderson]

I've just been looking at apache FOP and Scribus today.

It seems both contain a renderer from an xml vocabulary
to print-ready PS or PDF.

That is pretty much *all* FOP does; it is a command-line too.
The great thing about Scribus is that it also has a layout gui
for managing that xml declaration file.
I can't find any documentation for Scribus' native xml file;
I just see programmatic parsing in scribusXml.cpp.

Are there elements/attributes declared in the scribus xml
that are not supported by FO?
Or vice-versa?

Are there architectural reasons for Scribus to disregard FO?
(I notice that Passepartout also uses its own yet-another xml
vocabulary, and its own renderer.)

Are there features in Scribus's renderer that make it
superior to the renderer in FOP?
It seems both have to worry about hyphenation, color, fonts,
and embedded images.

Thanks for any insight; I'm just trying to wrap my head around
the different technological possibilities in this space.

-mda