Hi, to the best of my knowledge, this means that the digital multichannel format Dolby Digital is supported. If a multichannel track is presented on a DVD (or even on a CD, I guess), it is properly decoded and sent to the appropriate L/R/C/LR/RR/SW channel.
DPL2 is for turning stereo tracks into surround sound by guessing in a smart way what should go where.
Neither the 2900 or 5900 will have this feature. I have the 2900 but can check the manual again if you want.
I gladly give my opinion for a better one.