[habosa]

Not always. The kill system call can be used to send any signal, it's just got a name that implies you're sending something like SIGKILL or SIGTERM. I have written C programs that use kill for harmless inter-process communication.

[eru]

SIGUSER is quite common for that. Or SIGHUP.

[brokenparser]

By convention, SIGHUP is used by daemons to signal a change of configuration.