What is this thing - slider bar in bottom right corner

Hi,
I recently upgraded to 8.7.0 and I mainly use the people tags to categorise my photos. 
I've noticed that in the bottom right corner of the screen there is a slider bar that seems to be always grey-out but the value changes periodically. It seems to always about 30% but varies by 5 or 6% either way.


I've looked in the help, but as I'm not sure what I'm looking for, it does not help much.
What is this measuring?
I've also noticed (not sure if its new in 8.7.0) that I can now categorise photos by things other than people. An interesting one for me, is aeroplanes and DigiKam did a great job of finding them all and even sorting by similar types of aeroplane.Thanks for that one.
Thanks..Paul Kean