Minecraft wiki entry for Chunk, associated text: For example, when a generic mob wandering AI is executing, it asks if a certain block has a solid top surface. As part of that check, that chunk has an "unknown" ticket with level 33 placed on it. Similarly, commands such as /clone, /data, /execute (if|unless) (block|blocks|predicate), /fill and /setblock generate a temporary ticket when reading data from or writing it to a previously loaded chunk; if run every tick, they can prevent the chunk from unloading until the world is reloaded.[1]