Do you mean the sharp lines between the good quality and very noisy lighting?
If that's the case, are you using LightMix here, or some other render element?
I am guessing that what we see here is the result of using the adaptive light solver. To disable it and get more uniform results you can go to Render Setup > Performance and disable the "Adaptive light solver" option at the top.
This will most likely result in slower rendering, though. Even with the adaptive light solver enabled, you should be able to get good quality renders if you wait a bit longer.
