Space Beleren

Space sculptor (Space Beleren divides the battlefield into alpha, beta, and gamma sectors. If a creature isn't assigned to a sector, its controller assigns it to one. Opponents assign first.)
[+1]: Creatures in each sector can be blocked this turn only by creatures in the same sector.
[−1]: Put a +1/+1 counter on each creature in the sector of your choice.
[−5]: Destroy all creatures in the sector of your choice.

Unfinity

A creature can never be in more than one sector. If Space Beleren is on the battlefield, another Space Beleren coming under a player’s control won’t affect any creature’s sector assignment.

A creature that changes controllers maintains its sector assignment: its new controller can’t give it a new one.

Any time Space Beleren is on the battlefield, the battlefield is divided into three sectors: alpha, beta, and gamma. All creatures will be assigned to one of the three sectors.

As soon as there are no permanents with space sculptor on the battlefield, all sector assignments are lost. Notably, this means that if Space Beleren’s +1 ability has been activated earlier in the turn, creatures are no longer restricted from blocking, so watch out! If another (or the same) Space Beleren returns to the battlefield, creatures on the battlefield at that time will once again be assigned to sectors, but their new sector assignments are not influenced by any previous sector assignments they may have had.

For the last two abilities, you choose the sector as the ability resolves.

Sector assignment isn’t copiable. If a creature enters the battlefield as a copy of a creature in alpha sector, for example, its controller may assign it to any sector.

The sectors are essentially designations that change how Space Beleren’s other abilities will affect creatures on the battlefield. The sectors have no other effect on gameplay. Specifically, unless Space Beleren’s first loyalty ability has been activated, sector assignments have no effect on combat. A creature in one sector can block an attacking creature in another sector.

While Space Beleren is on the battlefield, any time there are creatures on the battlefield that aren’t assigned to a sector, those creatures are assigned to a sector by their controllers as a state-based action. First, all players who don’t control a Space Beleren (or another permanent with space sculptor, but come on) in turn order assign their creatures. Then all players who do control a Space Beleren in turn order assign their creatures.