Selection -> Sample Loop

Set the loop points based on the selection.

Create Loops in the Audio File Editor in Logic Pro — Apple Support

Selection → Sample Loop: The selected area is used to set the loop start and end points.

Create Loops in the Audio File Editor in Logic Pro — Apple Support

The Audio File Editor Loop commands, available from the Edit menu, are ideal for use with Sampler.

You can also use these functions to generate audio files that contain the loop settings in the file header. This allows you to make full use of these files in any app that can read the loop information in the file header.

  Increase Number of Transients  ⌃=

Increase the number of transients. The latest (10.6.1) online documentation incorrectly states that command-plus is the keyboard command. It is control-equalsign.

Edit audio files with transient markers in Logic Pro — Apple Support

A transient detection process is started for the audio material around the clicked position, and the transient with the highest energy becomes valid and is marked. If no transient is found in the catch area, a transient marker is created at the clicked position.

Edit audio files with transient markers in Logic Pro — Apple Support

Plus button (+): Increases the number of transients in the selected area of the audio file, or across the entire audio file. You can also use the Increase Number of Transients key command, default assignment: Command-Plus (+).

  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.

  Sample Loop -> Selection

I suspect that these commands will useful to people who used to use the ‘Apple Loop Utility’ which is no longer available. One of the more specialized tools/functions that Logic Pro X has available.

Create Loops in the Audio File Editor in Logic Pro — Apple Support

Sample Loop → Selection: The loop area (defined by the loop start and end points) is used to select a portion of the overall audio file.

Create Loops in the Audio File Editor in Logic Pro — Apple Support

The Audio File Editor Loop commands, available from the Edit menu, are ideal for use with Sampler.

You can also use these functions to generate audio files that contain the loop settings in the file header. This allows you to make full use of these files in any app that can read the loop information in the file header.

  Silence    ⌃⌫

Set the level of the selected audio to minus infinity — amplitude zero.

I am not sure of the difference (at least what it means to me) between Silence and Trim. Certainly with Trim a new audio file must be created when joining regions.

Seems like a destructive ‘Remove Silence’ command.

The ‘Remove Silence’ command behaves a bit differently depending on where the audio has been selected. If you select audio in the Tracks window the ‘Remove Silence’ command will replace the region on the track with newly-created regions. If you are working in the Project Audio window you will be given the option of replacing the region on tracks where it is used, or simply creating regions that you can use for other purposes.

Trim or silence audio files in Logic Pro — Apple Support

You can use the Silence command to set all amplitude values to 0, rather than removing (with Cut, Delete, or Trim) all data from the selected area of an audio file. This function is generally used to silence unwanted background noise in quiet passages.

Trim regions in the Logic Pro Audio Track Editor — Apple Support

You can trim an audio region in the Audio Track Editor to remove part of the beginning or end of the region.

Uses for Remove Silence in Logic Pro — Apple Support

You can use Remove Silence for a variety of different situations.