A Start Cue, strangely enough, is used to start other cues. Start Cues accept other cues as their target, which represent the cue that will be triggered when the Start Cue is fired during playback.
A Start Cue respects cue sequences. That is, when a Start Cue fires its target, the entire sequence of cues beginning at the target is triggered just as it would be during normal cue list playback.