OrthogonalExtensibility is the ability to extend the functionality of a message outside the body of the message. The ability to provide processing directives to actors or providers beyond or without extending what is contained in the body. A common use of OrthogonalExtensibility is using the mustUnderstand attribute to indicate that an actor or provider MUST recognize a feature and support it, or abort the processing of the message altogether.
Google on orthogonal extensibility.
See also : ExtraInterop