Whenever the system receives a trigger it cannot match the Unknown will be triggered. You can use this for fallback scenarios.
Our NLP engine learns based on the examples you provide for each intent. If there are specific queries the NLP should not match, you can add these as examples to the Uknown intent. This is especially useful in delicate situations like customer support.
Unknown triggers can be placed within the context of a flow. That way you can easily differentiate the reply actions based on the context of a flow.
Note that the unknown is also triggered when a user for example shares an image and you do not have a flow handling this specific situation.