Using more than 1 F9P on board

I have seen some guys posting boards with 2 F9P and also with 3 F9P.

Just wondering what is the reason of that? does it make fFix Position more accurate?

with 2 RTK receivers, you can calculate the orientation of the tractor (heading, roll, etc)

any documents or website I can get information about it?

yes, this one! just “search” or have a look at the “getting started” folder here