The Normalize effect module amplifies the selection as much as it can without causing clipping.
If you have a stereo (or any multi-channel) sound, you will normally want to normalize the whole sound, not just one channel, otherwise you’d amplify differently the channels, ending up with an unbalanced sound. This is why, if you select only one channel in a multi-channel sound, an alert will show up asking you if you really want to apply the effect to a portion of the sound, or if you want to extend the selection (and the effect) to all channels. If you click “Yes”, the selection will only be changed to include all channels: it will not be extended horizontally (in time).
