Use any Vector level as a live Clipping Mask.
No need to turn on Preview mode to see it or use Matte In/Out
Clip layers above/right of the mask to show only what's in the mask area or what's outside the area.
Clipping masks can be rendered while stil acting as a mask for another layer.
Since clipping masks are column based, you can animate the mask with the Animate or Skeleton Tools.