The Fade Out Audio Filter has two parameters: duration and type.
The type parameter was added in Shotcut 25.01.03
The following waveform images illustrate each fade of a sine wave over 1 second.
Natural: A log based curve (linear in dB). This type is considered the most natural in audio mixing and is sometimes called “Logarithmic”.
S-curve: Slow at the start, fast in the middle, slow at the end
Fast-Slow: Fast at the start, then ends slow
Slow-Fast: Slow at the start, then ends fast
For more complex fade types, consider using the Gain / Volume Audio Filter with custom Keyframe Types and Easing.