I only use Electric types. Help me
No, I know what those do. It’s the letter in white to the right of the set, in the Lusamine card it is an A.A/B/C is in reference to which block of sets said set is in. For instance, anything from SM1 to SM4+ (GX Battle Boost) are in the A Block. Everything from SM5 to SM9b are B block (considered the USUM sets I guess) are B block, and so on. It usually goes by year, and could very well have something to do with set legality eventually.