Another way to put it: NAC, Network Access Code, applies to P25 digital voice communications, and the NAC is the digital equivalent of CTCSS or PL codes. You will only hear the NAC coded transmission your radio is programmed to hear, while not hearing other transmissions that have different NAC codes.
CW ID is the analog morse code station identifier that is used to identify the transmitting station for purpose of FCC identification compliance. It's just station identification.
Technically speaking, not even police or federal users are exempt from station identification requirements so even public safety systems should be broadcasting station ID messages, whether in voice or CW mode, every so often, on each channel in use.