smart:limit is using the median of all measured PSR values (PSR = peak to short-term loudness ratio, see formula below) for measuring the dynamics. Although the PLR value (peak to long-term loudness ratio) is a more commonly used descriptor for dynamics, we found that our descriptor better matches the actual - also short term - dynamics of a track.
Our tests showed that the PLR value can sometimes be a bit misleading since it’s (basically) simply another descriptor for the integrated loudness. For example, if multiple tracks are normalised to -1dB True Peak, the PLR value is simply their Integrated Loudness value subtracted by 1.
PSR:
The PSR is the difference between the Short Term Loudness value and the maximum True Peak value measured within a window of 3 seconds.
PSR = Max. Peak - Short Term Loudness
=> computed within a 3 second window
PLR:
The PLR is the difference between the Integrated Loudness value and the maximum True Peak value measured over the whole observation period (e.g. typically the whole track).
PLR = Max. Peak - Integrated Loudness
=> computed over the whole observation period