The difference between Once-only alert and Continuous alert.

Once-only alert: Only trigger an alert the first time the alert threshold is crossed.
For example, if you set an alert for a temperature greater than 15℃, you will be notified when the detected temperature reaches 15.1℃.
If new temperature reading remains higher than 15℃, no new alerts will be triggered. When the temperature falls below 15℃, the alert resets and you will be notified when the temperature next exceeds 15℃.

Continuous alert: Trigger an alert every time the alert threshold is crossed.
For example, if you set an alert for a temperature greater than 15℃, you will be notified when the detected temperature reaches 15.1℃. Each time the device records a new temperature above 15℃, it will trigger a new notification.

Please note, the continuous alert can generate large amount of redundant alerts that can be annoying to certain users and also may raise concerns of spamming from email, SMS and push notification gateway providers. Therefore, when a continuous rule is triggered more than 5 alerts within the last hour, no further alert messages will be received until the next alert is triggered 1 hour later. The report icons in the Sensor Logs page will be displayed in red, illustrating the trigger has not been sent successfully.