8 years ago when the Underground was first opened, the indoor cycling bikes were pre-owned Schwinn and Star Trac models that had absolutely no data intel to mention, however, heart rate monitors were supplied to the cycling participants so we could project and monitor our intensity. I have so many great memories of those days, but I also remember that many participants - myself included :-( would then try and get to 100% of their heart rate (not a very safe goal).
Over the past years, technology has changed and we have arrived at where we are today with the Keiser M3i bikes and various activity trackers. We all have more information than we know what to do with. The Keiser bikes allow us to continue to project the data points from the class, and with everyone wearing an activity tracker we can start to pay attention to our heart rates as a tool to customize our classes. Although there are many activity trackers to choose from, the Polar Heart Rate monitor will be recognized by the bike. With that said, many of us have an Apple Watch to use for our heart rates. If I may offer you a suggestion of downloading the Zones app for your watch. I use it myself and find it to be helpful.
Now that we have the technology to capture the data, what do we do with the data? I found this article by Polar regarding the basics of heart rates that may help.