Hi Juan,
This relates to ambient calculations, and more specifically overture calculation. I had checked with Mostapha about this a few days back. Have a look at this presentation from slide #62 onwards.
The multi-processor functionality that we see with Radiance in Honeybee is actually a clever hack that Mostapha had written. Image-based simulations in Radiance only run on single core in Windows machines. The idea of running a multiple core simulation relies on generating multiple images and then joining them together. These separate simulations are prone to errors due to sync issues on Windows.
It makes sense to do the overture calculations on the entire image (hence a single core) as the nature of ambient calculations is such that diffuse inter-reflections from different parts of the scene will affect each other. Once we save the ambient file from the overture calculation, the final simulation can be run on multiple cores (in which the case the simulation is split into n # of separate image-based simulations which are ultimately combined into a single image).
Sarith