From what I know, it is most commonly associated with 5.1, but can include 1.0, 2.0, 4.0, 4.1, 5.1. With 2.0, it is a 2-channel carrier. The receiver may then process into 4 output channels. So what may on the surface appear to be 5.1, may in fact not be if more closely evaluated (e.g., is there a distinct center channel, or just front-speaker stereo?).
I don't recall my former Yamaha receiver as indicating the input source other than it being Dolby or DTS. But the Onkyo clearly indicates whether it is DD-5.1 versus DD-2.0. And I am seeing DD-2.0 for Lettermen. I will continue monitoring.