I'm not sure to understand but thanks for the answer!
I don't have a Low Level understanding of the hardware and my question was HLE related (feel free to remove it if it's not suitable here).
As you maybe know. Official SDK provide multiple ways to control the RDP. One of them is to set the RDP in 1 Cycle or 2 Cycles mode. In 2 Cycles modes, the RDP will write the Framebuffer twice with two different pipeline configurations (you send a DWORD containing two WORD, each of them contain the pipeline configuration for each cycle).
This "pipeline configurations" can involve a lot of inputs (colors, floats, texture, etc...) and define how they combine using the RDP pipeline (A-B)*C+D. One of them is "Combined Color". Most of the time, this "Combined Color" is used in one of the four entries of the second cycle. This make sense: The first cycle write the buffer, then the second cycle use the "Combined Colors" as input for the second cycle to compute advanced effects.
But I noticed LLE RSP emulators tend to ask for a "Combined Color" for the first cycle. This is as the framebuffer is not supposed to contain anything at this moment.
angrylion almost answer to this question stating RDP just use what "Combined Color" is supposed to be (aka "What is in the Framebuffer").
I'm not an hardware engineer but I wonder if there is not a way to control the sync of the RDP and some devs try to use this for some effects to do some 3 cyles + 1 cycle rendering, having a complex effect rendering (3 cyles) and a simple effect (the last cycle).
If this is out of the scope of CEN64, feel free to remove the post.