Phantom axes are virtual axes, they allow you to test your code while the network is unavailable or if your application uses more axes than you have available.
Phantom axes allow you to test applications offline or use more axes than are actually available on your network. You can add an arbitrary number of phantom axes to your network. Phantom axes can be added through RapidSetup as shown on gif below.
Note: be careful when transitioning between real axes and phantom axes. Some settings may need to be adjusted for your application to function properly when moving from one to the other.