Flashing lights may vary slightly from region to region but I don't think all that much.
Sure, if you are overtaking a vehicle a quick flash tells them you are coming up from behind and wish to overtake, They should respond with a single flash of the drivers side indicator which says- it is safe to overtake. However, if you are in the slow lane and an overtaking vehicle is safely past, a flash tells them they are clear to move back.
Conversely, if they respond with a single flash of the kerbside indicator then it is not safe to pass and you should tuck in behind until they tell you otherwise.
Once safely past we give single flashes L,R,L,R to say thanks.
Back in the old days of this country before CB radios we used to turn on our interior lights and hold up our log books to approaching trucks to tell them the 'scalies' were working down the line waiting to weigh us or check log books. Also repeated flashes of the cabin light meant the Highway Patrol were hiding down the line or alternatively, flashes of high beam are used to tell oncoming traffic that there is a speed trap.
Although there are cowboys out there who rely on them I would never recomend drug use to help complete a task-I have read that they are compulsory for US fighter pilots- and doesn't that speak volumes about the American military.
Stay hydrated with water not Coke- keep PK if you insist on candy but a bag of dried apricots is better.
I would appreciate someone in other countries passing on to me the codes used by truckers in their country.
Kind regards to you all and safe driving.
Happy New Year
Michael