It isn't clickable.
When I change settings to "hide engine lines" it disappears. That lead me to search for documentation on "Engine Lines". The main forum thread on this is https://www.chess.com/forum/view/general/engine-lines. However that didn't make sense as it implied there would be fewer lines when there's forced mate. As you can see below the number inside the icon is 99, which seems to imply that numbers closer to 100 are better.
It's the engine depth
It isn't clickable.
When I change settings to "hide engine lines" it disappears. That lead me to search for documentation on "Engine Lines". The main forum thread on this is https://www.chess.com/forum/view/general/engine-lines. However that didn't make sense as it implied there would be fewer lines when there's forced mate. As you can see below the number inside the icon is 99, which seems to imply that numbers closer to 100 are better.