RE: What does PCM mean on a transmitter?
Fwiw, I believe that the Multiplex 12-channel RX will only do 12 channels with a Multiplex TX in a special mode that uses a non-standard, higher frame rate. If you read the article posted above, you can see why it's not possible to have a PPM receiver with more than 9 channels with the current "standard" framerate. To get more channels, you have to change the timing between the TX and RX, and that means doing something that won't work with other systems.
I just flipped through the manual for the 12 channel RX, and it confirms that that Multiplex transmitters can use a special PPM mode with a faster frame rate to do 12 channels. The 12 channel RX looks for a signal, then decides if the signal is a "standard" PPM signal or their own mode that they call "PPM 12".
Really, in theory, you could do as many channels as you want in any mode, you just need to design your data encoding for it. It's just the "standard" PPM encoding used right now that maxes out at 9.