Coming from a Eurogamer interview with Assassin's Creed 4 associate producer Sylvain Trottier...

"The idea behind this feature is to be able to get the input of our players on their appreciation of each mission. The rating is anonymous and is sent to our servers to be analysed. It's an idea that came out of discussions between our creative director Jean Guesdon and our executive producer Sébastien Puel. It was implemented early in the production, more than one year ago. It's also being used during our playtest sessions to gather the player's feedback. It's fully optional. It's a way for our players to tell us directly whether they liked or not the mission. It's a survey if you prefer. It will help us know which missions are the favourites and which one people like less."

The following comings from Eurogamer Editor Ian Higton

"Each sequence was broken up into smaller story missions. After each story mission you are presented with a stats screen showing how well you 'synced' your memory of the event. On the bottom right of that screen was a small box containing five stars and the text: 'Rate This Mission.' By highlighting the box you could then rate the mission out of five. It took seconds to do and was not compulsory - sometimes I completely ignored it."

Trottier went on to say that future Ubisoft games might include a similar system of rating missions.