Dolby Digital Plus Primary Level Metadata Parameter and What It Does
"Primary Level" is a Dolby Digital Plus metadata parameter whose main function is to attenuate/duck the level of a Dolby Digital Plus Primary Audio Stream. In order for this parameter to have any effect, a Secondary Audio Stream must be present.
For example: Having director commentary (as a secondary audio stream) and the main program play simultaneously. When the director speaks, the primary audio will duck or boost according to the value specified for primary level. This field specifies a scale factor that should be applied to the program during decoding. Valid values are 0 to 63, with 0 interpreted as mute, and 1-63 interpreted as -50 dB to +12 dB of scaling in 1 dB steps. That said, the default value of "51" = 0dB or no gain change.