Rebel will analyze the book on the current level and add values to the moves of the book. This option is extremely handy in case you doubt about 2 or 3 book moves. With this you may consider taking the book move with the highest score!
The more time Rebel has to evaluate, the more reliable these values will be, and of course the better your book. In the submenu you can choose to make Rebel start again from the beginning or to continue where Rebel left last time this option was used.