A system is specified via a set of stories and specs, which are grouped in projects. Each project lives in its own shiny new website, where you can invite others and collaborate. As you will see, specifications and stories are simply wiki topics, with special annotations for messages, conditions, mocks and such, read more about it in Markup and DSL.
Stories, like a-lights-story, represent the requirements and test cases while the specs, like a-lights-spec, deal with the actual behaviour or implementation of the system - that's where the rules are defined.
The first thing to do to explore this project - here are some examples (or read first the Diesel Fiddle Guide):
See how you can use this tool.
Work on your current fiddles and stories.
Data samples: input/output etc.
Schemas for different data elements.
Domain categories: Admin Blog Category Data DieselCron++ DieselNode++ DieselStream++ Post Reactor ReactorMod ReactorModule ReactorTemplate Sample SampleCell++ SampleCpe++ SampleDevice++ SampleSector++ SampleTemplate++ Schema Spec Story TestAnnotated++ TestClass1++ TestClass2++ TestClassInv1++ TestClassInv2++ TestContainer++ TestEmptyBody++ TestSimpleClass++ TestSubtypeClass++ TestWithParms++ Topic