From the sysctl.conf(5) manpage:
sysctl.conf -- kernel state defaults
The /etc/sysctl.conf file is read in when the system goes into multi-user mode to set default settings for the kernel. The /etc/sysctl.conf is in the format of the sysctl(8) command, i.e.
Comments are denoted by a ``# at the beginning of a line. Comments can also exist at the end of a line, as seen in the EXAMPLES section, below.
/etc/sysctl.conf Initial settings for sysctl(8).
The following is a documentation project for the sysctl attributes on various versions of FreeBSD, starting with 6.2-RELEASE.