@superpokemon67: Your SID is combined with your regular trainer ID to establish "shiny spreads," meaning the particular seeds and frames on which shiny Pokémon appear, unique to your game. To add on with what's being said about the MAC address, that piece of information is taken into account when your game seeds itself in "Non C-Gear" (standard) format upon startup to eventually generate certain IV spreads on certain frames of gameplay (I believe it also directs PIDRNG seeding to get specific natures/genders/shininess, though I'm not 100%, it might be mentioned on the page). The new seeding variable was likely added to increase the degree of the game's "randomness" in the sense that now two different games can hit different seeds even if they're booted up at the same instant (which I suppose Game Freak felt made delay obsolete, though it's still present in C-Gear RNG abuse, explained on the Smogon page). Your MAC address is also combined with the region setting of your game, where even two of the same game in different languages will have a different seeding format.