You cannot set a processing mode for an event rule itself. If you add an internal event rule to a rule set, the rule set must have processing mode inferential. If you add an external event rule to a rule set, the rule set can be inferential or sequential. If you put an external event rule into a sequential rule set, the external event rule must be put in first place and must be the event that triggers the sequence. For more information about rule set processing modes, see About Rule Set Processing Modes.