Taken from the order book, bids and asks are categorized into five categories. Which particular category is based on how much a bid or ask deviates from the highest bid or lowest ask.
With a highest bid being valued at 10, a bid at 9.8 would fall into category 1% - 2.5%. Categories are divided between these percentages: 0%, 1%, 2.5%, 5%, 10% and 25%.
An aggregated version of the order book depth indicator is available as well. It works exactly the same way, but uses data of multiple exchanges. These exchanges can be configured.