How does the HotJar scroll heatmap work? [closed]

While analyzing the scroll heatmaps, I have come across confusing information about the scroll rate.

Sometimes the total scroll rate appeared above the average fold.

So what exactly scroll rate is taking in the count? - Is it the top or bottom border of the browser viewport? - Or is it something else? - Do all the heatmaps tools use the same definition of the scroll rate?