This answer came in the form of a private message. I elected to deal with it here as there are higher powers that can better explain this in simple and more technical terms. Basically, "S" pcm is 1024 resolution, "Z" is the older version and is 512. These two terms are as I said specific to JR.
PPM is known as FM (Frequency Modulation) and operates much like any FM signal. PCM handles instructions to the receiver in a much different way but to the average joe, basically does the same thing, that is, moving the control surfaces. I'm sure the gurus will chime in with all of the proper terminology. Bottom line is, FM works, PCM works, match the transmitter modulation to the receiver and go fly. As you become more selective and move to different levels in this hobby, it may become important to you to use PCM let's say in your large scale aircraft. Keep asking questions until it makes sense to you_bob