For further clarification; a list of events that can /haltdef (or /halt) with a ^ prefix can be found at:
/help Halting Default Text
Any event that uses the ^ prefix but is not listed there (like ON LOGON, or ON HOTLINK) will use it for different purpose, as mentioned. In general, "^" means "triggers before" (before the regular non-prefixed event), but in some cases there is no "default text" to halt/haltdef.
