Alex,
I can't point you to any documentation, but I can give you a bit of history from my fuzzy memories. First off, the goal of the runtime was to have a way to distribute BIRT free from the UI. The project wanted people to be able to include BIRT in their applications, but we did not want them to have to include all the UI components. So the Runtime is just those components required to Design (design engine API), Run, and Render reports with out any of the UI components present. When this was first created, it was tightly bundled with the whole eclipse platform, and required the engine to first start up the platform in order for the Report Engine to work.
Once the project got going, there was a lot of demand from our users to create something that was not dependent on the Platform. "We just want to drop the jars in the lib directory and run reports" was a common request.
So we modified the runtime to not be dependent on the platform, which is now the modern runtime. But what about users that have implemented BIRT using the 'old' way of doing things with all the platform code, plugins directory, etc. We didn't want to force them to change their code, or we wanted to let them just drop in their plugins. We also had customers that were deploying their reports within an RCP client. So that is what led to the Runtime-OSGI. It is simply a backwards compatible version of the Runtime.
Do we need to keep it? I don't know, my guess is no, but I would leave that up to the community.
Scott
