Signal flag "Quebec, " called the "Yellow Jack" is a simple yellow flag that was historically used to signal quarantine (it stands for Q), but in modern use indicates the opposite, as a signal of a ship free of disease that requests boarding and inspection.
a yellow flag, designating the letter Q in the International Code of Signals: flown by itself to signify that a ship has no disease on board and requests a pratique, or flown with another flag to signify that there is disease on board ship.
https://www.cnn.com/2020/02/09/us/coronavirus-under-quarantine/index.html