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Add and edit automation points in GarageBand on Mac
To create changes over time to volume, pan and effects settings, you add automation points to a track’s automation curve at different places, and then adjust the automation points so the value of the setting changes. You can adjust automation points by moving them up or down to a new value, or left or right to a new point in the timeline. Numerical values are displayed alongside each automation point.

Note: You can Use automation in the Piano Roll Editor in GarageBand on Mac to automate Smart Control events on a region basis.