Fade Out ⌃O
Create a destructive fade out on the selected audio. The selected audio will be reduced in volume (as defined by the fade curve) from the current level down to zero.
I prefer doing non-destructive fades on regions.
Add fades to audio files in Logic Pro — Apple Support
You can destructively fade in or out on an entire audio file, or a selected area in the Audio File Editor. Your changes are written to the file.