Need more detail on loudness box('ludt')

Hi team,

In this spec https://developer.apple.com/documentation/http_live_streaming/http_live_streaming_hls_authoring_specification_for_apple_devices

2.19. In fMP4 files, you SHOULD provide loudness information by way of a loudness box ('ludt'). When present, the loudness box takes precedence over any loudness information in the audio stream.

Here is a statement on loudness box. But this is not clear, what is the loudness box? How to set this information? Is there an example fmp4 file which can be the reference?

Thanks,