pin declaration

I see this topic has been covered. I have a custom built (by me) Arduino PCB. When i go to declare pins

pins 23 to 28 as digital can I use PCO - PC5 or am I required to use 23 -28?

That would depend on how you have pins 23 to 28 wired. In the Arduino support software, there is a lookup table that associates port bits with pin numbers, and it depends on the "board" definition.

pins_arduino.h exists place for each board type for the pin mapping. There are /variant folders for each board, you can find them there.