A: After processing, DePopper can show you a message like this: 3.2% samples exceeded max level in vol normalization. This means that for each 1000 audio samples (a standard audio CD has 44,100 audio samples per second), an average of 32 samples had to be corrected to avoid clipping (if the anti-clipping is enabled, which is the default) or were clipped (if anti-clipping is disabled). Ideally you should set the highest level which does not display this message, or displays a low value (the default value is calibrated to give good results in most situations).

Why this happens: normalization tries to bring all sources to the same average level. If the source has long passages with very low level alternating with some high level burts (for example, classical music), its average level is very low, but it has very high peaks. Therefore, its peaks can become too high after normalization, exceeding the maximum allowed level. The anti-clipping is very effective and because of its action usually there's no audible distortion, but the classical music listener can (not necessarily will) prefer to lower the gain/level in order to preserve the strength of high level passages as much as possible.

For non-classical music, values below 8% can normally be ignored without noticeable change in quality, but if you don't want any message just lower the "Level(dB)" in Normalize Volume Settings to -28 or even -32.