Minecraft wiki entry for Simulation_distance, associated text: Simulation distance defines a square region of chunks around the player where entities are ticked. Using distance of 6 for instance, entities move normally within a 13x13 chunk column around the player chunk. One more chunk out, within a one-chunk-thick square frame surrounding this region, redstone still runs, fluids flow, and crops grow (subject to normal chunk ticking rules). Beyond that nothing moves or changes. Using a simulation distance of less than 8 may cause hostile mobs to spawn in the outer frame non-entity-ticking chunks as long as the spawn position is still within a 128 block radius sphere around the player; however, because they are not ticked, the mobs do not move or despawn naturally and cannot be killed by command[2], even though they still contribute to the mob cap.[3] They despawn once the player moves more than 128 blocks away.