A solid yellow square, Quebec stands for "Q" in the signal flag alphabet. I feel like this one is shots fired at Quebec big time. Not many people take too kindly to being called yellow, and I don't imagine the Quebecois are any different. Sorry Quebec. Don't shoot the messenger.
You'd fly this flag if you were entering port and felt that you didn't need to be quarantined. It's basically a guarantee that everyone and everything onboard your vessel is healthy. This is a request though, so don't be surprised if you get a big fat November thrown right back in your face. It's easy to see this one being abused in the event that zombies take over your boat.
Check out the rest of our Signal Flag explanations!