The help text in the web UI's threshold editor reads:
high: Triggers when the value of the data source exceeds the "value", and is re-armed when it drops below the "re-arm" value.
low: Triggers when the value of the data source drops below the "value", and is re-armed when it exceeds the "re-arm" value.
This language implies that the comparison is greater-than for high, less-than for low, but in reality it's greater-than-or-equal / less-than-or-equal in the ThresholdEvaluatorHighLow class. We should clarify this point.
NB i have not checked the other threshold evaluator classes against their own descriptions in this help text. We should do that too.