I am trying to pick ram for my laptop. I can see these types of ram on the site.
Ddr4 2133 pc4 17000 Ddr4 2133 pc4 17000s Ddr4 2133 pc4 17000u Ddr4 2133 pc4 17066r Ddr4 2133 pc3 17066
What does the latter at the end or lack of it mean? Which one should I choose? Also, about the 3 or 4 after pc. What is it supposed to mean? Is bigger number better?