Even though the Business Rules Engine (BRE) is included with BizTalk 2006, it does not need to be exclusively called by the BizTalk host.  The BRE has a .NET API library that enables any host to communicate with the engine programmatically.
You must first reference the Microsoft.BizTalk.RuleEngineExtensions.dll from the BizTalk installation directory (the default installation is C:\Program Files\Microsoft BizTalk Server 2006).

You can then call BRE policies with the following code

Microsoft.RuleEngine.Policy policy = new Microsoft.RuleEngine.Policy(“PolicyName”);
Policy.Execute();

The execute method can take any number of objects as a parameter that need to be asserted as facts to the BRE for execution.