What is a Kernel Routine ?

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 ?