Comment 5 for bug 1561131

@be: As noted, dB is a relative unit, which expresses the amount of change respect another value. As such 0dBFS expresses the amount of change, versus a predefined level named "Full Scale" (Max digital level, or unity level). This unit only has sense in a meter, not in a fader.

In this case, the slider for the replaygain is indicating a change of gain versus the default target loudness, and usually people require increasing the gain ( i.e. push it nearer to 0 ). Just remember that the usual precautions about mixing gains should be respected.

There is no standarized unit to describe this. It could have been named 0dBRgT ( Replaygain target).
As Daniel says, what has been done to 1.12/2.0 is to express this gain in a more reasonably unit (at least in recent years) which is using the LUFS unit. The expression in dBs helps to know where the 0 point is and should imply the meaning that it is a gain change.