BDD Viewer
The bdd viewer provides the ability to story-test the functionality in your domain objects, using a behaviour-driven design (BDD) framework.
Currently the only framework supported is Concordion, though we expect others to be supported in the future.
Alternatives
The JUnit viewer can also be used for testing. However, with the junit viewer the actual test is written in JUnit (rather than XHTML as is the case for Concordion) and so is closer to a developer test than a story test.
A note on licenses
The bdd viewer itself is licensed under Apache License v2. However, the Concordion integration (naturally enough) depends upon Concordion, which is licensed under Apache License v2 but in turn depends upon Xom, which unfortunately has an LGPL license. Apache projects are not allowed to have dependencies on LGPL projects.
There is a workaround. Isis' pom.xml files specifically exclude the XOM dependency, meaning that they are compliant with Apache's licensing restrictions. However, any application code that uses the BDD viewer must explicitly add its own dependency to the XOM library. You'll find that the pom.xml files generated by the quickstart archetype do indeed do this).
If you are unhappy to introduce this dependency to LGPL in your own code, then you will not be able to use the Concordion integration.