From the plate to the driver, and no further.
You send to a plate
Scan it or type it, then pick a line or write your own. You are told the message was delivered. You are never told who owns the car.
Their phone alerts them hard
It buzzes, it stays on screen until it is dealt with, and a second report buzzes again rather than stacking silently. That treatment is reserved for being blocked in.
They reply, or they come
The conversation belongs to the incident, not to either of you. When it is sorted, the messages are deleted within a day and you are both warned first.
Not registered? It still works.
Your notice prints with a code, and that code holds the plate for thirty days. Whoever scans it finds your message waiting.
Retired codes, closed accounts and codes that never existed all answer identically, so scanning one tells a stranger nothing.