Habe mal das beim schnellen googlen gefunden:
To check which points are missing, it is most advisable to check the "progress tracker" ingame, which mentions the side objectives which have to be completed to achieve 100% sync. 100% completion is solely tied to everything mentioned in this progress tracker.
These include all the different collectibles (Animus fragments, chests, secrets (which includes manuscripts and letters), viewpoints, shanties and the Mayan stones). Also required are the unlocks of all taverns, the defeat of the 4 legendary ships, the liberation of all forts, the completion of all assassination & naval contracts as well as the templar hunt missions.
Also ich würde als erstes mal im Progress Tracker schauen (weiß jetzt nicht wie der auf Deutsch heißt, sollte aber zu finden sein )