The mixing elbow is where a marine diesel's hot exhaust meets the seawater that cools it. Hot gas and salt water meet inside a cast iron bend, and the inside corrodes and fills with carbon until the passage narrows or the casting cracks. SAIL calls it a consumable, and it fails on every engine sooner or later.
How a surveyor spots it
From outside, rust bleeding from the casting and soot or salt crystals round the joint with the exhaust hose. With the engine running, bubbles or a drip at that joint mean gas and water are leaking. At the outlet, a weak stream of cooling water. SAIL adds an engine that won't rev to its usual maximum, because the narrowed elbow chokes the exhaust.
Below it, the bilge. An owner on the YBW forum points to unusual water in the bilge and corrosion beneath the manifold as the early signs. Then the paperwork: an elbow with no record of being changed on an engine with a few thousand hours is due, whatever it looks like outside.
What decides the grade
A leak. Gas or water escaping from the elbow or its joint is an A, because of carbon monoxide in the boat or water back into the engine. Corrosion with no leak on an elbow past its interval is a B. Carrying a spare is a C, and a common recommendation, since owners report failures starting as a drip that gets you home if there's a spare to fit.