Minecraft wiki entry for Updating_old_terrain, associated text: The plugin inspects each chunk of your world, and decides for each chunk whether it can be deleted. In Minecraft, a single chunk can contain different biomes. For example, along the shore of an ocean, a lot of chunks will contain ocean biomes as well as beach biomes. Sometimes it is not obvious how these cases should be handled, so the plugin can be used in one of two modes: