A databook is a book containing statistics for manga or anime characters. This is usually done to provide extra info and backstory to characters and is usually done for shounen manga. Most databooks are considered secondary canon and as such are no match against manga scans.
Some databooks are unreliable because it's either not written by the author or contain info disproven by scans and manga. Case in point: the Naruto Databook.
Various Narutards were known in the past to use the hyperboles from the databook to cover up the fact that Narutovese was pathetically weak against other verses and then justify it in a ridiculously illogical roundabout way.
Great debates have taken place with other databooks such as Dragon Ball's Daizenshuu, most recently in the 4 galaxy controversy, which Dragon Ball Super eventually solved for itself.
Other databooks, such as the Bastard!! Databook are more reliable. Nevertheless, they are always regarded as secondary canon and take a backseat to any contradictory information seen in the primary canon.
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