Q:  12.19.18  7:57 pm my bad, i meant to ask when is c g b d G/C and when Cmaj7 add 9 without the third?
A: whichever way you think your reader(s) will understand it better/quicker. i always write G/C cuz i know what that is and i use it all the time. many people are used to reading slash chords, but those who are not may get them backwards. maybe some people would prefer to read Cmaj9no3 which looks long and crazy to me but i'm sure some people are used to seeing that