I'm grokking the HFS linux standard documentation, and I can't move beyond this concept.
The concept I don't get can be demonstrated by the following quote:
The contents of the root filesystem must be adequate to boot, restore, recover, and/or repair the system.
Firstly, maybe I'm misunderstanding what a "root filesystem" is. It sounds like it is the specific "/" directory, and all subdirectories.
But then the following statement seems to clash with the original point:
"The primary concern used to balance these considerations, which favor placing many things on the root filesystem, is the goal of keeping root as small as reasonably possible. For several reasons, it is desirable to keep the root filesystem small. ... It is occasionally mounted from very small media."
If you mount " / ", you're mounting the entire system, no? (Perhaps the documentation means that we put the different directories in " / " on different partitions, ie putting /bin, /sbin in the "root partition", while "/usr, /home, /media, /mnt" elsewhere. Then why is "/usr, /home, /media, /mnt" included in the root filesystem and not in an independent system?
Again, this goes back to the original question: What else is there to a system other than a root filesystem? What use is it to put data "not in the root filesystem", if it's just going into a subdirectory?