Why ?
The system under development is a bridge between your Fixtures and the system your testing.
If you want to change the way
GreenPepper is finding/instanciating your fixtures, or if you need to hook the document execution then you can define a Custom System Under Development.
Changing how
GreenPepper is finding your Fixtures
This could be useful when you are using for example an IOC, like Spring (see Spring Example) or just when you want to add locations (packages in java, namespaces in .Net) for
GreenPepper can resolve your fixtures.
To change the system under development :
Only for specifying location to resolve fixtures (packages in java, namespaces in .Net)
By this custom system under development you tell
GreenPepper to look in "com.mycompany.fixtures" and "com.mycompany.specials.fixtures" to resolve fixtures in specfications that your are running.
Changing Fixture instanciation mechanism (Spring example)
Hooking document execution
Nice work! btw, I have already seen smth similar at http://rapid4me.com
but didn't download - decided it was smth unworthy. I was really mistaken.