What’s the purpose of a black rubber ring near the bottom of a drain body?
Ring labeled &quot;A&quot;:

Multi-function tapered rubber gasket that centers the drain assembly in the bottom sink hole, cushions the clamping action of the nut that secures the drain assembly, and seals the lower hole (most sinks have an overflow drain-hole and channel cast into the body of the sink that leads to the space between the upper main drain-hole and the lower main drain-hole, overflow liquid enters the drain assembly through small holes in the drain assembly within that space).

Ring labeled &quot;B&quot;:

This is a plastic (sometimes metal or fiber) friction ring; contradicting it's name, it prevents friction between the nut and the rubber gasket above so you can firmly tighten the nut without the nut grabbing and twisting/turning the rubber gasket.