A key describing a gain to be applied so that the output loudness matches the Target Reference Level. This is typically used to normalize loudness across program items. The gain is derived as the difference between the Target Reference Level and the Program Reference Level. The latter can be given in the bitstream and indicates the actual loudness value of the program item.
The value is given as an integer value between 0 and 127, and is calculated as -0.25 * Target Reference Level in dBFS. Therefore, it represents the range of Full Scale (0 dBFS) to -31.75 dBFS.
This key is only used during decoding.