From QB64 Wiki
ON creates event procedure calls or enables event trapping.
- To turn on event trapping for ON COM (n), ON KEY (n), ON PEN, ON PLAY (n), ON STRIG (n), ON TIMER (n) and ON UEVENT.
- In the case of ON ERROR the trap is also enabled until a subsequent ON ERROR statement.
- ON procedures should be used only in the main program module and not inside of SUB procedures!
- If you have used the $CHECKING:OFF metacommand, $CHECKING:ON will turn on c++ error checking again.