yellow flag required to be flown when vessels enter a foreign port. "I request free practique" meaning all on board are healthy and no plague carriers. :-)
I have flown that signal flag many times when sailing into French and C.I.ports
It,as I have mentioned,an indication that No communicable disease is on board the vessel and a request for free passage ( Free Practique) to enter the port without hinderance.
I believe they have otherwise there must of been some very worried people on shore when I sailed into a Port displaying my Q signal flag on the Flag halyard!
:-)