You can assign any connection as power output (+5V) or ground.
You need to be aware of the following limitations:

• Power and ground pins can only source/sink 30mA max current per pin.

• Total current draw must not exceed 500mA

If you need more current than 30mA then you can use one of the fixed power pins on connector J7. See the U-Config program board image for details of which pins these are. In fact it is good practice to use these pins whenever possible for a 5 volt source as they are powered directly from the USB supply.
Using assigned pins instead offers advantages of modular connectors and is particularly useful for optical devices which always need a supply for the optical switches and dont have a large current requirement. As each modular plug has one fixed ground connection you would rarely need to assign a ground pin.