Warning (Horn or Flash)

A sudden sound makes you look (if you can hear) at the source of noise.  That's the job of your horn.

Rolls-Royce owners' handbooks used to say, "Use the horn like a polite cough, to announce your presence".

Use it sparingly (rarely), when another person doesn't know you're there. Never use if after they've seen you.

Flashing draws attention, like a lighthouse. It means the same as the horn but is more effective in noisy surroundings. Other uses cause distraction, or confusion: if two people see it they might both move into the same space at the same time. Say "Thanks" just by raising your fingers off the wheel.

If another car flashes at you, look at its position, speed, wheels.  Only those will tell you what it's doing.  Then you can decide what to do.