Scott, that is the black tank flush as indicated in the warning above it - (Not well labeled I agree).
Here is a diagram of what the black tank flush does. After you drain the tank, attach a hose to the flush and turn on the water and it sprays out the tank more or less. Don't leave it running with the drain valve closed, or you might get a surprise when it's full (flows out the vent), but I do close my drain valve to let it fill a bit sometimes if I'm not distracted.
I use a separate gray hose for that from my potable hose (avoid cross contamination), and when you shut the water off and disconnect, disconnect from the water source first because it needs to drain back from the check valve up high and you will get wet if you disconnect from the tank flush inlet first.
Also, if you install a clear pipe adapter between the sewer outlet and your hose (available camping world), you can visually see when you have flushed enough. I use a straight one like this because my sewer outlet is already adequately angled downward. Clearview 5" Sewer Adapter with Bayonet Lugs - Valterra T1021 - Sewer Fittings & Adapters - Camping World