/etc/securetty is used by login(1); the file contains the device names of tty lines (one per line, without leading /dev/) on which root is allowed to login. Bering default file looks like:

# /etc/securetty: list of terminals on which root is allowed to login.
# See securetty(5) and login(1).
#
# Include ttyp0, ttyp1, etc to allow telnet access. *NOT RECOMMENDED*
tty1
tty2
tty3
tty4
tty5
tty6
tty7
tty8