I'm guessing (for instance) you would mean you consider an AM transmission on a particular radio station as one signal - which is different to that being transmitted on another radio station.
I would clarify this somewhat ....
In reality that (AM) signal can include a range of frequencies - say within a 10kHz spread either side (upper and lower sidebands) of the center carrier frequency for the particular station - depending on the station content modulation signal characteristics.
And it's possible to imagine (again - for instance) two AM radio stations transmitting on the same carrier frequency and the two signals arriving at the same receiver - it could end up a mess for the listener. Hence the need to regulate who gets to use a particular part of the RF spectrum and under what conditions they can use it.
Ever tried using CB when the trouble makers got going?