This command fades audio, i.e. smoothly gradates its volume from one level to another. If a selection exists, only the selection is faded, otherwise the entire document is faded. The command displays the Fade dialog, which lets you specify the type of fade:

InFade from silence to full volume.
OutFade from full volume to silence.
CustomFade from one arbitrary level to another: in this case the starting and ending levels must be specified, in decibels.

To make the fade logarithmic instead of linear, check the Logarithmic checkbox. To do the fade, click OK or press Enter.