Selection -> Region ⇟

Redefines the length of the region.

Select audio in the Audio File Editor in Logic Pro — Apple Support

Define the selected area using a region

In Logic Pro, select a region in the Browser or main window, then choose Edit > Region → Selection.

This function is useful if you want to reselect the entire region for cycled playback—after performing a number of edits, for example.

The reverse can also be done, allowing you to redefine the length of an existing region by choosing Edit > Selection → Region.

You can modify selections by changing their start or end point, or you can move the entire selected area completely.

Note: If you choose Edit > Snap Edits to Zero Crossings, any length adjustments to audio region start or end points snap to the nearest zero-crossing point of the waveform.