In his post entitled Business Rules, OCL, XML and Schemas Daniel Cazzulino writes

DonXML is proposing extensions to OCL to express business rules that can be used at code-gen time and at run-time. He mentions my Schematron implementation called Schematron.NET, which allows many business rules to be expressed simply in terms of standard XPath expressions. I believe such an XPath-based language is good enough to express almost every business rule.

Udi Dahan commented as an example, a rule "only a bank manager can authorize a loan above X" which he said couldn't be expressed with Don's idea. It could, indeed, with something along these lines (XPath-like):

<assert test="sec:principal-role('BankManager') and po:Loan/@Amount < 1000">
  Only a BankManager can place a loan of more than $1000.

Using rules-based XML validation is a good way to augment the capabilities of the W3C XML Schema language which is traditionally used to describe message structures in SOAP-based XML Web Services. In the post on Daniel's blog Udi Dahan asks

I like the technique. I'm still puzzling over the strategy. From a SOA approach, where does this go ? What makes it different/better than any other rules engine ? You've given me something to think about. Thank you.

In an SOA approach the rules are part of the message contract. A service endpoint can accept certain kinds of messages that satisfy its message contract. Using a rule-based language like Schematron just makes for writing a tighter contract than one could write using a traditional XML schema language like XSD.

In fact, Aaron Skonnard wrote an article on MSDN entitled Extend the ASP.NET WebMethod Framework by Adding XML Schema Validation  that introduced this to some degree which he followed up with two episodes of MSDN TV; Validating Business Rules with XPath Assertions, Pt. 1 and Validating Business Rules with XPath Assertions, Pt. 2