I have a series 8 Apple Watch. I believe when setting up your Apple Watch, if you enter that you have been diagnosed with atrial fibrillation, it processes data somewhat differently. It will provide periodic reports of your afib burden, and will not alert you whenever it senses irregular heartbeats. It also shows you what time of the day your afib burden was highest as well as which days of the week, although I'm not really sure how useful that information is? Regarding afib burden, the lowest level it will report is "2% or less", even if your afib burden is 0.