VU meters added to Cambridge Audio

Cambridge Audio recently added vu meters to certain newer products via firmware update. My 851n streamer is too old, but the EVO 75 I use in my television setup now can display vu meters. Very cool.  Here’s my question — what are they actually measuring. They don’t seem to respond to clime changes and continue to dance even if volume is down to zero.  Are they waiting input levels? For fun only?  Are all by meters like this during playback? (The last vu meters I had were on a real to real in the 70s.). I’ve submitted to CA as well, will update if they reply.