pCP 4 Real Time Kernel vs pCP 3 Audio Optimized

I am still running on a Rpi3 the "old" 3.22 version since it is great.
I never used the Audio Optimize version before because of issues with Rpi3 WiFi

What is the difference between the old Audio Optimized and the new Real Time Kernel?
When is suggested the standard and when the Real Time Kernel?

Thanks a lot.