When fluxbox installs, a pointer in /etc/X11/Sessions is not created. This file is the short name of the window-manager (eg. fluxbox or gnome) and contains one line that 'points' to the location of the window-manager binary (eg. /usr/bin/fluxbox). This allows you to specify 'XSESSION=fluxbox' in /etc/rc.conf. It isn't necessary because X will search all bin directories, but it would be nice to have it in case the binary ever moves.