NOTES
From FreeBSDwiki
The NOTES file lists all the kernel options available for your version of BSD and your specific architecture. You should look at it if you're going to be building a custom kernel, to see what sorts of goodies you might like to include.
There are actually two separate NOTES files; one resides at /usr/src/sys/conf/NOTES and details all the non-platform-specific kernel options for your version of BSD, while the other resides at /usr/src/sys/(arch)/conf/NOTES and details all the platform-specific kernel options for your version of BSD and your particular architecture. "(arch)" can be i386, amd64, ia64, or (whatever you architecture happens to be).