uname -r and patchlevel

[n dhert]

Can somebody explain about the -p<level> one sees in the output of the
uname -r ?
Under  *exactly* what conditions the patch level changes to a new value
after you applied a freebsd-update install ?

Does -p<level> only change if
 a) a change of the file /boot/kernel/kernel was part of the update
or also if
 b) *some* /boot/kernel/* files (but not /boot/kernel/kernel itself)
    changed as part of the update
Will in any case a new -p<level>  be shown only after a reboot is done ?

I have confusing differences in -p<level> increase/not increase in different
FreeBSD systems after applying freebsd-update install.
The handbook is too vague about what conditions *exactly* will make uname
-r  show the newest -p<level>.