2.1.37 [ISO32000-1] Section 12.7.8, XFA Forms
The specification states:
PDF 1.5 introduces support for interactive forms based on the Adobe XML Forms Architecture (XFA). The XFAentry in the interactive forms dictionary (see Table 218) specifies an XFA resource, which shall be an XML stream that contains the form information. The format of an XFA resource is described in the XML Data Package (XDP) Specification (see the Bibliography).
XFA forms are not supported (their presence is ignored).