You can use same input or output ports for multiple external effects or instruments. This is useful, if you work with multiple analog effects and synthesizers, and your audio interface has a limited number of inputs and outputs.
The input and output assignment for external effects/instruments in the Audio Connections window is non-exclusive, that is, you can use the same port for input or output busses and for multiple external effects and instruments.
Note
However, if you work with external instruments or effects, and you record audio at the same time, make sure to use dedicated device ports.