The Toggle Source/Record in Timeline button allows you to view multitrack source material quickly in the Timeline for selecting and marking specific tracks.

By default, the Timeline displays only the available tracks for source material.

When you click the button to display the source material, both the button and the position indicator turn green to indicate that you are viewing source material.

This feature is particularly useful when you are editing with a sequence or subclip created from a sequence; you can also use it to look at the contents of any source clip in a Timeline display.