This is used to mark the ending frame for selecting a video segment.

In a 3-point based editing control system used in the Editor, a selected video segment is the basic object for editing operations such as Cut and Copy.

Visually, a short vertical line immediately below the video player’s slider window indicates the MarkOut. The exact location of the MarkOut can be obtained from the Goto MarkOut push button located below the Time Code display window --- the one with a similar bitmap like the MarkOut but with a different color.