Flagging
I believe the term originated from the days when the Royal Navy ruled the waves.
Royal Naval ships would visit ports in the Empire, with all flags flying, to remind the 'natives' who was boss. The only reason for the visits was a show of power and was known as 'Showing the Flag'. The term later evolved into meaning a temporary stop or transit.
Within the RAF Air Transport fleet it was changed to 'flagging' and meant what Ex-Ascoteer has already posted.