If you have been diagnosed with AFib you should enable AFib History

If you have paroxysmal atrial fibrillation (afib episodes that come and go), then Apple Watch’s “Afib history” setting will be helpful.

Each week, it estimates the percentage of time you spend in afib, which is helpful to evaluate whether your rhythm control medication is working and map out your individual triggers (coffee, stress, exercise, and so on). The Afib history feature is FDA-cleared.

The only drawback is that when you enable the AFib History setting, irregular rhythm notifications get disabled. However, those notifications aren’t recommended for people with diagnosed afib – Apple’s irregular rhythm notifications algorithm was designed and tested to screen for irregular rhythms in a population without diagnosed arrhythmias.

So TL;DR - turn it on!

I have ‘lone aFib’ where i get episodes every few months or years, and the aFib history setting is basically useless - it’s always under 2% of the time so nothing is reported. If you have more frequent episodes i could see it being useful though.