In the 'forums.php' you have:
'In phpBB3 there are no categories, everything is forum based. Each forum can have an unlimited number of sub-forums and you can determine whether each may be posted to or not (i.e. whether it acts like an old category)'
but in the 'install.php' file:
'During the installation process your first category and your first forum are assigned an appropriate set of permissions for the predefined usergroups administrators, bots, global moderators, guests, registered users and registered COPPA users. If you also choose to delete your first category and your first forum, do not forget to assign permissions for all these usergroups for all new categories and forums you create. It is recommended to rename your first category and your first forum and copy permissions from these while creating new categories and forums.'
and in the another place - 'Your first category'.
I don't know - in the phpBB3 are 'category' or not.