where else are such signs used?
In broadcasting and radio communications, and railroad stations
Call sign
In broadcasting and radio communications, a call sign (also known as a call name or call letters—and historically as a call signal—or abbreviated as a call) is a unique designation for a transmitter station. In the United States of America, they are used for all FCC-licensed transmitters. A call sign can be formally assigned by a government agency, informally adopted by individuals or organizations, or even cryptographically encoded to disguise a station's identity.

The use of call signs as unique identifiers dates to the landline railroad telegraph system. Because there was only one telegraph line linking all railroad stations, there needed to be a way to address each one w
what is apollo 9? It was a human spaceflight, the third in NASA's Apollo program. who were the crew members? McDivitt, Scott, and Schweickart did they use any call signs? Yes, "Gumdrop", "Spider" etc.
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Call sign [SEP] Introduction
In broadcasting and radio communications, a call sign (also known as a call name or call letters—and historically as a call signal—or abbreviated as a call) is a unique designation for a transmitter station. In the United States of America, they are used for all FCC-licensed transmitters. A call sign can be formally assigned by a government agency, informally adopted by individuals or organizations, or even cryptographically encoded to disguise a station's identity. The use of call signs as unique identifiers dates to the landline railroad telegraph system. Because there was only one telegraph line linking all railroad stations, there needed to be a way to address each one when sending a telegram.