As we know, Moscode uses the set of dash and dot to represent the character. However, during the crisis, they don't have enough time to send the long code to say "Please help me" or "I am in a big trouble."
Therefore, the military ends up with the easiest Moscode signal set which is S.O.S. to represent the message "help" or "we are in trouble." S.O.S. can be understood by only Dash-Dot-Dash. This way they can send a help message much faster.
Since then, S.O.S. is getting popular and used in other purposes.
Flag = A piece of cloth used as a symbol, standard or signal of something
Stand for = Represent
Complicate = Become complex
Telecommunication = Long distance communication
Some sources from http://www.thefreedictionary.com
*Each word can have more than one meaning. The meaning here is just to favor the article
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