Grease / Lubricants

Hey guys this being my first Side by Side I am trying to stick with the Pure Polaris products for maintenance. So I understand the oil and oil filter, I guess I understand the different lubricants for the Gear Cases (Though to be honest I do not understand why the one they make for the main will not work in both). BUT TWO DIFFERENT GREASES. If the one for the U-Joint is a must why can’t I just use that one for everything? Can someone help me with this am I missing something?