Futaba 3pm question

I just bought one of these radio and have been trying to figure out the same stuff.
I don't really know the diference between hrs and ppm but, You can switch between the two modes.

In ppm mode you set the fail safe each time you turn on the Tx by pushing the button on the back of the Tx. It only effects the the throttle (Ch 2) and has no effect on steering. I don't really like that you can't permanently set the fail safe.

When it's in HRS mode you set it through the menu. Only problem is that some of the brushless ESC won't work with HRS.

I use a 3PK (FM), another of the so called computer radios - these transmit using 3 different kinds of user selected modulation schemes (oldest to most recent) - PPM - Pulse width modulation, PCM - pulse code modulation and HRS - high response system. These are diff ways of encoding/decoding the info on the FM signal. They get faster in the same order with HRS being the fastest. I would assume the 2.4Ghz system is doing the same kind of thing - they are two diff ways of modulating the signal.