Modboat is correct, there are a number of videos on YouTube which explains this.
As default these are model- flysky01-flysky02 -flysky03 ect ect.
Each receiver needs to be bound to a seperate model number on the transmitter, (number the receivers). You can rename these models to registration numbers on your boats, or their names.
The binding process is quite simple, bind key plugged in to bind port, power 4.8-6v battery pack ready to be plugged into any other channel. Switch transmitter on while holding down the bind button on transmitter. Then plug in power to the receiver, after a few moments you'll see the red led on receiver stop flashing and go solid red. These are now bound. Unplug bind key power everything off, power up transmitter then receiver. Test with a servo.
Select a new model on transmitter, (press and hold cancel to save change). Then repeat above process.
Jon