A Shuusei is another word for a correction, a shuusei is drawn by the Animation Director of the episode and sometimes if you are lucky you will find one done by the Chief Animation Director... It depends on how hands on both directors are and how important the image is for a full correction. A lot if shuusei are partials of a hand or small detail that needs fixing. Shuusei are drawn on a rainbow of colored paper, and generally will have a higher quality than the other sketches in the set.
A partial shuusei/correction sketch from the film Vampire Hunter D: Bloodlust.
Animated GIF showing a close up of the correction (yellow paper) sketch laid over the corresponding genga (white paper).