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

the sampling and quantisation of an analogue domain signal into digital samples

those sample quantisations alone in the digital domain i.e. in memory or storage i.e. .pcm files

the transmission of quantisations or indeed any bitstream as a digital signal with a digital line code through a DAC into the analogue domain

the process of 1+2

the process of 1+3
I like this paper a lot: The philosophy of PCM. It explains PCM very clearly, includes references to some of the earliest papers on it, and you can hardly argue with the qualifications of the authors :)
PCM was conceived as a three-step process. First, sampling (at frequency $f_s$ sufficiently above Nyquist). Second, quantizing (with enough resolution $\Delta$ to obtain the required quantization SNR). Third, coding, or mapping the quantized samples to $M=2^k$ pulse amplitudes, with $M$ chosen to obtain the required line pulse rate.
PCM, then, has parameters $f_s$, $\Delta$ and $M$. Each of these affect system performance in different ways.
Why use the term &quot;modulation&quot;? In communications, &quot;modulation&quot; is very general, and is used almost as a synonym to &quot;mapping&quot; or &quot;converting elements from an input set to an output set&quot;. In fact, what you understand as &quot;modulation&quot; (multiplying by a carrier) is often called &quot;upconversion&quot; and &quot;downconversion&quot;, to avoid confusion.