The extract below is copied from PDF 2.0 specifications:

Interactive forms (12.7), also called AcroForms, were introduced in PDF 1.2 and serve as a collection of
fields for gathering or presenting variable information interactively from the user. The XML Forms
Architecture (XFA) was introduced in PDF 1.5 to address the need where the number of fields needs to
vary depending upon the quantity of entries, that can be accommodated through XFA’s use of a form
schema (which is called a “template” in the XFA specification). XFA forms were deprecated with PDF 2.0.

If an interactive processor supports XFA forms, that processor shall clearly indicate to the user when
they are interacting with an XFA form that they are interacting with an XFA form.

The introduction of dynamically generated pages guided by schemas is a considerable departure from
the fixed page model of PDF files designed to address use cases where dynamic forms are a strong
requirement and therefore this architecture is documented separately from PDF (XFA reference).

EXAMPLE An invoice form may need to provide a variable number of invoice item fields ranging from 2 or 3 to hundreds or thousands. When the exact number of invoice items is determined, using the directives found in the XFA schema, a specific set of pages can be created to accommodate the required number of fields.

The implementation of such a schema driven page generation involves considerable effort beyond that
for a simple PDF viewer and therefore a PDF processor may choose to not implement this feature.
The following clauses describe how XFA is integrated into PDF, where the schema (template)
description can be found within the PDF, and where the various types of XML data can be found.
Whether or not the pages generated by the processing of an XFA schema are materialized as PDF pages
is implementation dependent.