All right. Look. This mechanic appears on one card, and is incredibly unlikely to ever show up again except on possible reprints of that one card, so this is getting covered purely for the sake of completion.
So, with that out of the way: Space Sculptor appears on the Planeswalker Space Beleren from Unfinity. Once Space Beleren is on the battlefield, all creatures who are already on the battlefield or who enter it from that point on gain a sector designation – alpha, beta, or gamma. Those designations are chosen by the player who controls each creature, starting with opposing players in turn order and ending with the player who controls Space Beleren. Creatures entering the battlefield get a sector designation as the enter. The sector designations last for as long as that creature is on the battlefield, or until Space Beleren is no longer on the battlefield.
Got all that? When Space Beleren enters the battlefield and Space Sculptor becomes active, each creature gets assigned to one of three sectors. All of Space Beleren’s Planeswalker abilities then affect all of the creatures in one of the sectors, and have no affect on creatures in the other sectors. Noncreature permanents don’t get assigned a sector and stay on the battlefield as normal, although if any of them become a creature, they must then immediately be assigned a sector and will retain that designation even if they stop being creatures.
And that’s it. Sectors and designations have no rules significance beyond Space Beleren, the only card with Space Sculptor, which only appears in an Unset. While Space Beleren is technically legal in Legacy, Vintage, and Commander, I would not ever expect to see this exact mechanic on another card in the future. Something like this ability with a few tweaks? Maybe, but I’m not holding my breath.
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