Minecraft wiki entry for Command_Block, associated text: A button to the right of the Previous Output text pane‌[JE only] or the left of the "Previous Output"‌[BE only] specifies whether the last output should be stored and displayed. It is O‌[JE only]/enabled‌[BE only] when the output text should be stored and X‌[JE only]/disabled‌[BE only] when the output text should not be stored. For worlds with many command blocks, especially command blocks running on fast clocks, not storing the output text can reduce memory and storage requirements.