If you create a number box and then open the "properties" window there is a section that says "messages", "receive symbol", "send symbol"

Could someone explain what this means?

Receive and send symbols broadcast or pick up float messages so you don't need wires.

For example, if you create a fader and set SendSymbol = thisfader then you can pick up movements somewhere else in the patch with [receive thisfader]

OK, thanks!

So the symbol you choose is supposed to be a new one, to identify this particular "channel" of communication? I had been thinking it might be the name of the quantity you want to send/receive.

Yeah, any unique identifier will do so long as it doesn't start with a digit.

Of course, it makes sense to name your control channels meaningfully, ie use the name of the
quantity is fine. If the name of the slider and the send symbol are the same there won't be a clash, the name is just a name.