Tauno Voipio schrieb:

ICAO Annex 10, Volume 2, part 5.2, Distress Communications.
The rules come from ITU Radio Regulations.

It's part 5.3. And the phrase "mayday relay" is *not* defined there.
It's always dangerous to cite a source without having actually read it.

There are several differences between maritime and aviation radio
procedures. One is that the expression "mayday ralay" is not used in
aviation. Another example is that in aviation you say "stop
transmitting" rather than the maritime "silence". Let alone the maritime
procedure to get a clearance for climbing to FL200...