It's just a reminder to order a CR 1620 battery for the TCU to have on hand for when you start getting the error message on the TCU. It's a battery size that is not very common in neighborhood stores. If it's never been changed it will eventually become a problem if you ignore the TCU alerts but the Garmin/Wahoo alert is not critical. I proactively change it every year or two. Xacto knife and rare earth magnet is the easiest to remove the battery. The manual suggests tweezers but it's the worst tool to use.