Shows the price of BTC on multiple exchanges, helps to see which is leading in price and if the current price is above or below average.
For use on COINBASE:BTCUSD unless you customize it
To use on other platforms (Bitfinex, BitMEX, etc) go to options and enable "Show Coinbase" then hide whatever exchange you're currently on.
To disable exchanges, uncheck them under the Styles tab.