What is the meaning of the “yellow cards” and what is special about some of them. In story the wave

Mr. Ross passes out little yellow cards that he calls "membership cards" . He tells his students that if their card has a red X on it, it means they are a "monitor". Monitors are supposed to tell Ben if any members of The Wave aren't following the rules. This alludes to ways the Nazi's kept watch and control over people during WW2.