What is a Kernel Routine ?

Question

In another answer I met a short overview of the sections of the man pages:

1. commands for $-User
2. system call, that means evoking a function from the Kernel
3. library call, that means evoking a function from a conventional file
4. special files
5. conventions and configuations
6. games
7. miscellanous
8. commands for #-User

Now mostly there were mentioned these 8 sections, but a few times there was also mentioned a 9.th section:

9. kernel routine

What is the very meaning of a Kernel Routine, what is its purpose within the operating system ?

Solution

The Linux-based system I tried doesn't have a section 9 in its man page, but, looking online, one can find the NetBSD intro(9) man page:

This section contains information related to the internal operation of
the system kernel.  It describes function interfaces and variables of use
to the systems and device driver programmer.

This appears to be about APIs provided by the kernel.

Answers to the following questions may be relevant: what is a Kernel?