MSFS is capped at 30FPS on both the Series S (1080) and X(4K) . Depending on VRR implementation it will either match refresh rate to your FPS or to a multiple of your FPS, this makes the game appear smoother it does not actually change your FPS. It is my understanding that if you are using a VRR screen MSFS on the Series X will unlock the FPS from 30 which may or may not get you higher frame rates depending on where you flying and what you are doing, you will rarely if ever get 60FPS.
Certified means that the company paid money to have the screen tested and it meets certain standards and they are allowed to use the logo. In theory you are getting a better screen, although a none certified screen might be identical, the manufacturer may have just not paid for the testing and right to use the certified logo.
Biggest difference between HDR10 and HDR10+ is how many nits of brightness are used 1000 vs 4000.