1) STEP's powerful EXPRESS schema language, offers a well-developed constraint
     formalism for defining product data.
2) XML’s support for the Web could greatly facilitate the interchange of
    STEP-conforming product information. STEP data could, of course, be
    interchanged using XML, given an appropriate XML-based information architecture
    that is widely understood and accepted.

One group wants to fully incorporate XML as an interchange format between STEP-compliant databases. To be used in this way would require a standard means of expressing the entire data model of a STEP-compliant database, together with a subset of its contents, in the form of valid (or at least well-formed) XML. The interchange package would presum ably need to include appropriate DTDs
(if any), Link files, and a robust universal addressing mechanism. The inclusion of stylesheets would presumably be optional, depending on whether, in addition to data interchange, the contents of the package were intended for display.

The other group does not think that XML is up to the task of fully interacting with STEP. This is because they think it is wildly unlikely that generic XML browsers can usefully support interactions with STEP source data. There are far too many specialized semantics. And if you can't usefully interact with STEP source code using a generic XML browser, what's the advantage of using XML to interchange STEP data? They instead suggest using SGML/HyTime for interacting with STEP source data. XML should be used separately to providing access to STEP data on the Web.

It is not clear which of these views that is “correct”. They both agree that there definitely is a place for XML in STEP, but they disagree about how it should be used.

Bibliography :
a study of eXentensible MarkupLanguage, 
Pontus Norman