Force your users to classify their pages, ensuring each document in your instance has been classified. When this is turned on, users will be forced to select a classification level when creating/editing pages that do not already have a Classification Level set.
This setting can be changed at the global and space level.
Enabling it at the global level means that you can turn it on/off for the whole site.
Settings > Compliance Configuration > Classification > Global Options > Force Classification Globally
Doing it at a space level allows a more granulated control i.e. only enable it for specific spaces while disabling it for the rest.
Navigate to a space > Space tools > Apps > Compliance > Force Classification