A scene that is to be rendered is called a render job. Render jobs get split up into one or many tasks that are rendered on the render nodes. The number of tasks a job is split into depends on its resolution and samples per pixel.
Instead of starting with an octree and an emittergrid, scenes can also be created from region files. ChunkyMap can use this so it doesn't need to construct the octree on a Minecraft server. New jobs that are created from region files are put in the region processing queue first and added to the render queue after a region processing node has created the scene files.
When a task is done, it needs to be merged with the part of the job that is already done. This is done by a single dump processor that also has a queue of dumps waiting to be merged.
If a job gets cancelled, it is not removed from the queues. The nodes will check if it is cancelled and just skip them in that case.