The major distinction between a static sensor and a dynamic sensor is that a static sensor always exists and a dynamic sensor can come and go. Dynamic Sensor Basics and Dynamic Sensor Using Multiple Metrics, explain how the sensor creation happens. As a quick recap, the sensors get created when their creation criteria is met. For example, when a temperature exceeds a threshold. This means that when looking at the sensors at run time, that instead of seeing everything, you only see the problematic ones.
But what do you do when the temperature drops below the threshold. Do you continue to show the sensor or remove it? There is not a specific answer, it depends on the role of the sensor. In the example where the sensor was highlighting the weather for fires, when the temperature drops below the threshold it makes sense to remove it. But what if the sensor was the temperature inside a refrigerator and being above that temperature means the food has spoiled? Just dropping below the required temperature doesn't fix the spoiled food so you would want the sensor to remain.
This decision to stay or go away is based on the settings in the sensor evaluation tab
When Expire dynamic sensor is unchecked, the sensor will never be deleted once created. As noted above, there are various use cases where this is valuable. For example, the sensor was the vault door being opened during off hours. Just because someone closed it again, it still might be something you want to know.
When Expire dynamic sensor is checked, the sensor will exist only as long as its health is below 1.0 (100%). By default, severities Warning and above have health < 1.0 (this can be changed in sensor properties, health tab). Using the temperature example, the sensor would get created when the temperature threshold was exceeded and would be deleted 20 seconds after it went back below the threshold. There are many use cases that benefit from this logic, such as exceeding the number of messages backlogged. Once the backlog is below the threshold, the sensor is not needed.
The interval allows for a situation where the condition oscillates quickly. That is, the temperature is hovering around the threshold. The sensors waits a bit rather than constantly creating and removing the sensor. A key element here is that the time period needs to be larger than the frequency that the data changes. If the temperature is only reported every minute, an expiration of 20 seconds would still remove the sensor before it could be updated. For this, a value of at least 70 seconds would be more appropriate to allow for another temperature update before expiring the sensor.