Use the Roto Brush tool any time you would have used traditional rotoscoping. It’s particularly useful for removing a foreground element from the background.
The segmentation boundary is the boundary between the foreground and background. The Roto Brush tool adjusts the segmentation boundary as you progress through the frames in the Roto Brush span.
Use the Refine Edge tool when you need to rotoscope objects with fuzzy or wispy edges. The Refine Edge tool creates partial transparency for areas of fine detail, such as hair. Use the Refine Edge tool only after you’ve adjusted the segmentation boundary across the entire clip.