This level of abstraction appears to be due to the organization of
the W3C divided into independent working groups. Since the charter of
the W3C XML Schema Working Group is hidden on the private
members' only section of the W3C web site, we may
think it did not include the definition of the processing model of a
schema validation and only focused on the definition of the language
itself.
However, the effects of this lack of formal specification are similar
to those of the unpublished APIs practices that some software editors
are famous for: the lack of a concrete description is an obstacle for
most users to understand the PSVI, and it creates a kind of
"vendor lock-in," since generating
the PSVI using another tool instead of W3C XML Schema involves
emulating these unspecified APIs and may prove difficult for many
developers.
Four years of XML have taught us that there is an easy way to
serialize abstract concepts, and the definition of a XML
serialization for the PSVI would have a lot of advantages. Assuming
the format is simple enough, it would let us visualize what the PSVI
is, allow us to process a PSVI using the standard set of XML APIs
(DOM, SAX, and friends) and tools (including XSLT), make it easy to
include into XML processing pipelines, allow us to save it for reuse,
and permit us to generate it out of any application or tool able to
generate XML documents.