PCM is a digital format and not every channel is sent in every packet. E.g. the number 886 (expressed in binary) in a particular packet type and position means move the aileron servo to #886 of 1024 possible positions (steps) (for PCM1024). I recall that Futaba PCM is not completely (or only recently) reverse engineered.
PPM and PWM are analog formats in that you can use non-digital components and get an "infinite" resolution (no steps) in the signal sent to the servo. This is limited though by noise, jitter and the bandwidth of the carrier actually used. Typically though these are implemented with microprocessors and have a fixed number of steps that is based on the resolution of the timers the processor uses. For instance if the processor uses a 1 microsecond timer like LP and other do for the slower signal formats, then you get 1000 steps between 1.0 milliseconds and 2.0 milliseconds and the familiar min=1000 and max=2000 in the PWM output setup.
PCM could be used from Rx to FC, but it isn't; mainly because it is proprietary and different for different manufacturers. A PCM radio always puts out it's signal in some other format like PWM, PPM, SBus, etc.
PPM even has different RF signal formats where JR/Spektrum has opposite polarity compared to Futaba/HiTec