The term Pixel Pitch is a distance between the center of two adjacent LED’s. e.g. P10 pixel pitch LED screen would have a 10MM distance between the two cluster of LED’s. Each cluster of LED would consist of three RGB (Red, Green, Blue) LED’s in a conventional screen and same applies for the SMD screen.
Viewing distance is the minimum distance from the screen for optimal viewing; P10 screen would have 10 Mts of minimum viewing distance and so on.