Pins labeled as NC are not connected. The package pins/balls are floating. 

However, care must be taken on a device with NC if future pin migration is considered. On a lower density device with the same package, when moving to a higher density device, the pin could be used in that same package. Otherwise, if the future pin migration is not considered, or if the same pin is also NC on all devices in the family using this package, then the pin can be used for breakout routing, if needed.