Going down some serious worm & rabbit holes here, @cosmiclibrarian.
There's good jargon in semiotics. A 'sign' is that which conveys meaning, with the construct of 'sign' including the various elements of signifier, signified, and interpreter.
The conveyer mechanism (independent of sender and receiver) could be called a message, letter, telegram, siren, musical note, traffic light, etc (things to be 'read'). Time Travelers could use some kind of signaling device, ie, a signal.
@cosmiclibrarian
A msg is usually thought of as a statement. This 'signal', a toot on Mastodon, or a postcard, for example, carries with it greetings, news, and other kinds of info.
When thinking about ET contact, such as @setiinstitute, what is important is the signal itself. It doesn't need more info to be momentous. The signal exists. Somebody sent it. Somebody is out there.
Like a fossilized footprint, the 'sender' may reveal no other details. What's significant is that they were there.
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