TdOPCEventSubCondition provides one substate of a TdOPCEventCondition.
A TdOPCEventCondition can be single-stated or multi-stated. A multi-state condition is one whose state encompasses multiple ranges or sub-states which are of interest. For example a "Level Alarm" condition may have multiple substates (subconditions) including "HighAlarm" and "HighHighAlarm". Each subcondition is represented by a TdOPCEventSubcondition. Each subcondition is associated with an TdOPCEventCondition. The subconditions of a multi-state condition must be mutually exclusive, e.g. a tag cannot be in both "HighAlarm" and "HighHighAlarm" at the same time.