Flags are default labels in Freshdesk. They appear in the ticket list view and inside the ticket details page when certain activities occur:

New: This flag shows on a ticket when an agent's response to the customer is pending. It remains until the first response/resolution isn't overdue.

Customer Responded: This flag displays when a customer responds to a ticket.

Overdue: This occurs when the ticket surpasses its resolution time without being resolved.

Response due: This flag appears when the agent's first response isn't sent within a specific time based on the SLA.

These flags are default and hardcoded, unalterable or removable. Ticket filtering using these flags isn't an option as of now