S2S radiation model
 

Hello. I have a problem here.

I'm working on a natural convection problem using ANSYS fluent. When considering radiation model, I found that there are two S2S model can be selected (As shown in the figure 1).

My question is:
1) Which should I choose and what's the difference between these two models?
2) In other of my cases, sometimes there would be two S2S model but sometimes only one abvailable. How does this happen?

Thx a lot for any response!

Figure 1:
https://ibb.co/31qzG47