It is for the digital out. It is a trs to RCA, the RCA would be the digital in of a dac, like the Hugo.
Many times on daps you get some wrap around, so that the imaging to the right and left tend to be in your ears. The DX90 keeps this out in a more flat plain, were the sounds should be coming from, so you have a nice wide soundstage and one that keeps recorded sounds out where they should be. It also maintains a nice natural depth, if the recorded that way.