Sweep Start and Sweep Intro are segue styles that allow you to play a sweeper or short audio element over the intro of a track. The key difference is in timing:

Sweep Start makes the sweeping track start at the same time as the next track, provided it's short enough to finish before the intro point ends. If the sweeping track is longer than the intro, it will start early and play up to the end of the intro period.

Sweep Intro makes the sweeping track finish playing immediately before the end of the intro point of the next track, timing the sweeper to end precisely when the vocals begin.

To set up either style, you need tracks with Intro or Early Out cue points set (these can be configured via Manage > Tracks). In the Segue Editor, click on the waveform to access segue information and select either Sweep Start or Sweep Intro from the segue style dropdown. Both styles will automatically use the Early Out cue point from the previous track if available.