Abstract Types and Substitution Groups
They best way to describe these is to use an analogy from object-oriented programming. You can define a base class, then define subclasses, all with the same named method. Then you can process objects of the subclass by referring to them as if they were all members of the base class. This type of polymorphism allows you to call a named method but have different behaviors because each of the derived classes has implemented the named method in a fashion tailored to the derived class. Abstract types are akin to the base class, and the members of a substitution group are akin to the derived classes. I've been told that this approach is used in the OASIS Security Assertion Markup Language (SAML), but I've seen ...
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