Everyone starts with an SPR of 50. When you win it goes up, and when you lose it goes down. How much? That depends! Beating somebody with a higher SPR will reward you more than beating somebody with a lower one, while losing to an opponent with a higher SPR won’t punish you as much as losing to a lesser opponent. At the same time, a big win or loss with a differential of 15 points will affect your rating more than a narrow 1 or 2 point win or loss. (If you’re familiar with Elo rating system, it’s like that!)