This is a general indication of a well made dough. You get the white ring because the doughnut floats in the oil and that part of the dough does not come in contact with the oil even when the doughnut is flipped. If the batter is too thick, or hasn’t risen enough it sinks deeper in the oil and you won't get that ring. Cake donuts are heavy, so they won’t have the line, but yeast raised donuts always will. If your yeast raised donuts are sinking and cooking up tough, try proofing them longer before you fry.