An excerpt from page 32 of BI's Help:
"Introduction" => "User Interface" => "Timeline view and controls"
"......The timeline view shows a horizontal time-based view of both clips and alerts together.
“Tracks” are created based on camera colors. As a clip may contain many video start/
positions, a timeline rectangle represents the entire time covered by one clip, but this does
not indicate continuous recording over the period represented. However, triggered alerts
are represented by a lightning bolt icon above the tracks and an orange band beneath the
tracks to represent the time during which the camera was triggered. If you are recording
only when triggered, these orange bands therefore will represent times during which video
was actually captured...."
More on Page 109, "Clip Playback and the Viewer Window" => "Timeline Playback"