1) Click the star mark on the upper left of the thumbnail in the thumbnail pane.
The clip is registered to [Favorites].
Click the star mark again to unregister it from [Favorites].
Drag and drop the clip in the thumbnail pane to the [Favorites] on the sidebar.
The number of star marks can be changed by clicking the star mark in the properties. Star marks can be set up to 5, and clips can be searched by the number of the stars.