What _specifically_ is PCM? I've seen it refer to: 

1) the sampling and quantisation of an analogue domain signal into digital samples
2) those sample quantisations alone in the digital domain i.e. in memory or storage i.e. .pcm files
3) the transmission of quantisations or indeed any bitstream as a digital signal with a digital line code through a DAC into the analogue domain

4) the process of 1+2
5) the process of 1+3