You must have heard of the system log, which keeps track of a wide variety of things that have happened on the system. Transaction SM21 reveals the system log, which will hold information about dumps, failed login attempts or even debugger change actions in which the variable that was changed is outlined. Want to make your own mark ?
The system log is for system-wide matters. Thus it is not the first type of logging an Abap developer should be concerned with. It's a very prominent place to appear and as system logs are monitored very well, this is where irrelevant logging is unwanted logging.
I haven't found matters relevant enough to hand to the system log myself - so far. A serious candidate has been a failed payroll run, but as this is not a fully automated process, failing steps are noticed straight away.
Write your own
Function module RSLG_WRITE_SYSLOG_ENTRY is the Bapi to write an entry on the system log. Use it for system wide matters only, intruder detection or reporting improper use, core-business issues or show-stoppers. Make sure you don't write too many entries, refer to Businss Applicaton Log if there is more detail to log.
Just for the record: I have tried this module - to see if it works as expected, but I have yet to find a suitable implementation purpose. Whatever it is you are developing - the system log is too high up the hierarchy.