call sign; distinctive letters
Primarily a maritime term for a ship's unique identification code, often used in radio communication.
A ship's call sign is registered internationally.
The distress signal included the call sign.
Borrowed from English, more general for radio call signs, while 信号符字 is specifically the written code letters for ships.
Compound of 信号 (signal) + 符字 (code letters), a technical term likely coined for maritime use.