The Restoral Margin is used for the upper and lower thresholds (e.g. Temperature & Humidity) and requires the values to cross back over the threshold by a certain amount before a new event is reported. This prevents excessive event messages if the measurement is at or near the threshold.
For example, consider an upper temperature threshold set at 30 degrees Celsius and the restoral margin set at 5 degrees.
If the temperature initially exceeds 30 degrees then an event is generated and a message is sent to the network.
The temperature must now drop to 25 degrees and then again exceed 30 degrees before another event is reported.