The Oracle 12c 2 Day DBA manual states:
While whole database backups can be an important element in your overall backup strategy, they are also a required step in some situations, such as when you enable or disable ARCHIVELOG mode.
(I'm assuming that a "whole" database backup is the same as a "full" database backup as mentioned in the Concepts section.)
In the section "Enabling Archiving of Redo Log Files" the manual states:
You cannot use backups from before the switch to ARCHIVELOG mode to restore and recover the database to a point in time after the switch. Thus, if you do not immediately make a backup after switching, then you are running your database without a valid backup.
Is the first excerpt of the manual wrong? Is the requirement that you need -any- type of backup? Or does it really need to be a full backup, as opposed to a level 0 backup? If so, why the difference?