K script execute at shutdown or when you change run level.
for just an example if you set postfix link K100postfix
in runlevel3 and when you switch any runlevel to runlevel 3 it means postfix service stopd.
when you shutdown or restart system scripts of /etc/rc.d/rc0.d will execute and all script links (exclude killall and halt) start with K
you can compare it yourself
ls -l /etc/rc.d/rc3.d
ls -l /etc/rc.d/rc0.d