"Part of the o/s" is not the best description. The idea is that there is a clear demarcation between all of the programs that come as part of the operating system, and all of the programs that are applications softwares that are installed on top of the operating system.
* https://www.freebsd.org/cgi/man.cgi?query=ps
ps has one set of options and has used getopt() since April 1990.
* https://www.freebsd.org/cgi/man.cgi?query=inittodr&sektion=9
Documenting the operating system encompasses documenting the kernel as well.
* https://docs.freebsd.org/en/books/handbook/
"Part of the o/s" is not the best description. The idea is that there is a clear demarcation between all of the programs that come as part of the operating system, and all of the programs that are applications softwares that are installed on top of the operating system.