Start Next will only start the next sond (thus the name) but not fade out the current song.
You must use the Fade Out marker instead.
The fade will go from Fade Out to Cue Out (or the end of the file if no Cue Out is set).
The Standard Fade Duration is used when a song is faded out prematurely, e.g. when it’s faded out by an event/fixed time, or when you click the NEXT button during automation.