Minecraft wiki entry for noise_settings, associated text: vertical_gradient a conditional surface rule to select blocks in a gradient, used for deepslate and bedrock (between the specified y-coords is the gradient itself)
 random_name:[needs testing] any random name.
 true_at_and_below: the lower y-coord where this condition is true.
 above_bottom: the y-coord above the bottom of the world, mutually exclusive with absolute and below_top.
 absolute: the exact y-coord, mutually exclusive with above_bottom and below_top.
 below_top: the y-coord below the top of the world, mutually exclusive with above_bottom and absolute.
 false_at_and_above: the upper y-coord where this condition is false.
 above_bottom: the y-coord above the bottom of the world, mutually exclusive with absolute and below_top.
 absolute: the exact y-coord, mutually exclusive with above_bottom and below_top.
 below_top: the y-coord below the top of the world, mutually exclusive with above_bottom and absolute.