I have a hard time finding specifications or a DTD, RNG, or XSD for XFA.
Adobe lists the "Adobe Supplement to ISO 32000, BaseVersion 1.7, ExtensionLevel 3" on their PDF Reference page. On page 102, there is a link to the "Adobe XML Architecture specifications", however that link is dead, and archive.org did not archive it.
There is an old W3C Note "XFA-Template", which has a link to a DTD, however that link is dead as well, and from archive.org I get the feeling it's never really been alive. (In any case, XFA-Template only seems to be a part of the XFA family of specifications.)
A Google search for "site:adobe.com xfa specifications" turns up a link to "Introduction to XML forms architecture (XFA)". However, there is a DNS problem for me and I can't access it:
$ traceroute6 help.adobe.com traceroute: unknown host help.adobe.com
Archive.org doesn't help here either.
Any hint where to find that information?
I found a copy of the 3.3 Spec here...
I'm not sure what your goal is but I wouldn't be investing much time in XFA. It's been removed from the PDF 2.0 specification.