During a conversation, a Microsoft Copilot Studio agent triggers the appropriate topic based on the user's input. If the agent can't determine the user's intent, it prompts the user again. After two prompts, the custom or Copilot agent escalates to a live representative through a system topic called Escalate.
You can customize how the agent behaves when user input doesn't trigger any topic. For example, you can build a catchall topic to capture unrecognized user intent, create a topic to call back-end systems, or route a topic to existing services.
Customize the fallback topic and behavior in the default system Fallback topic.
A fallback topic triggers On Unknown Intent to capture the unrecognized input.
Reference:
https://learn.microsoft.com/en-us/microsoft-copilot-studio/authoring-system-fallback-topic