PPM is analog. Servo's position is directly mapped to how long does the signal last. It is a direct mapping. HRS is using digital technology (maybe base on PCM technology). The transmission signal is fully digital.
The reason why digital servo can only be use for HRS is its power driver design is ready for high flame rate power cycle. Just like conventional ESC and current high-freq ESC. In each signal frame, HRS will give the power to the servo in a serial of smaller cycle. While PPM will give a one but longer last cycle. This means, a servo for HRS must have the ability to withstand such fast on-off cycle. And the servo designed with such ability is call digital servo. Details please refer to www.futuba-rc.com