Kitchen sink developed a leak. Fixed by tightening washer under sink in basket waste. To do this obviously had to disconnect basket waste from trap. Became clear when reattaching that other washers need replacing as well
That’s fine but when looking at the waste pipes I noted that the sink, waste disposal and dishwasher joined to a common waste pipe 40mm as expected. However just before it left the cabinet this pipe was joined by a smaller one. There is no obvious source for this smaller pipe. What is it likely to be? Could it be relief in case waste pipe obstructed? I attach image to clarify.