Dolby Digital Plus (E-AC-3) provides up to twice the efficiency of Dolby Digital while adding new features like 7.1-ch audio, support for descriptive video services, and support for Dolby Atmos (but more on that later). Dolby Digital Plus is widely used by streaming and broadcast services to deliver surround sound audio at lower bitrates. 5.1-ch audio in Dolby Digital Plus is typically encoded at bitrates between 192-256 kbps while stereo audio in Dolby Digital Plus is typically encoded at bitrates between 96-128 kbps. Dolby Digital Plus bitstreams are not directly backwards compatible with Dolby Digital decoders, but Dolby Digital Plus decoders can decode Dolby Digital bitstreams.
Source:
Dolby Audio over HDMI part 1: Codecs | Dolby Developer