If you are using Radiance in any of its flavors (Accelerad, HB-Legacy-Daysim, HB[+] etc), then it is backwards raytracing.
Back in 2017, while working on the back-end code of HB[+], we had validated/tested HB[+] against standard raytracing simulation results. (Well, I see that you’ve seen this paper on researchgate, but leaving it here just to complete my explanation!). I didn’t perform a physical validation of the approach, firstly for the lack of time, but also because my assumption was, and still is, that I did likely be retracing the steps from John Mardaljevic’s 1995 study where he validated Radiance against physical measurements by using a Perez Sky Model.
The climate-based sky generated by Honeybee and HB[+] employs gendaylit, which generates a Perez Sky Definition. The daylight coefficient approach, variants of which are employed in Daysim and HB[+], uses a combination of sunpath representations and discreteized sky domes to approximate the Perez Sky definition. What I found, and has also been replicated in tests by a few other people, is that HB[+] performs (marginally) better because we consider more precise sun-positions than Daysim does.
I am not sure if you can get away with using the same values for ad in Daysim and HB[+]. Daysim employs the classic Radiance algorithms involving ambient caching in its raytracing method while HB[+] employs pure Monte-Carlo approach.
In the ambient-caching approach, one can somewhat control the precision of the simulation by setting low enough values for ambient accuracy (-aa) and high values for other complimentary settings such as -ar, -ad etc. Radiance will keep sampling and tracing rays till the prescribed level of accuracy is reached. In pure Monte Carlo approach, there is no guarantee of any precision as the number of rays traced is set by the user. So, there is a risk that if you set low values for -ad you will end up tracing less rays than required.
In my opinion, one way to check if you are running both Daysim and HB[+] to their maximum possible precision levels is to keep cranking up rendering parameters till the results stop changing between successive iterations (i.e. convergence).
Finally, John Mardaljevic has been conducting a tutorial session on ambient calculations every couple of years at the Radiance workshops. The slides from that should provide you with some idea about what is going on under-the-hood with regards to calculation parameters in case of classic Radiance/Daysim. Unfortunately, there are no complimentary explanations for the Pure Monte-Carlo aspect of things, besides this one presentation from Greg from 10 years ago.
Regards,
Sarith
(PS: In case you aren’t already, I would highly encourage you to follow the relevant research by Eleonora Brembilla on this topic.