Minecraft wiki entry for Altitude, associated text: Altitude expressed as the bottom face of a block layer is most useful for identifying the position of blocks, such as when using /fill to replace block(s). Expressing altitude as the top face of a block layer is more useful for identifying the position of entities, such as when using /tp to teleport or /summon to spawn an entity/entities. For instance, the topmost block of water in an ocean biome has a Y-coordinate of 62; in a frozen ocean biome, ice replaces water at sea level, and the lowest height where terrestrial mobs will spawn is at layer 63.