You have multiple options to specify Action(s) in a Rule for a Conversation, but let's point out the difference between two seemingly similar Actions:
Close Conversation
Use Close Conversation Rule Action if you want to indicate that no other follow-up or communication is needed with the Customer. It means the issue/question is solved.
Mark No Reply Needed
Use No Reply Needed Rule Action if the issue/question is not solved but may not require a reply from an Agent.
For example, if the Customer replies with "thank you" to acknowledge the last message the Agent sent about further researching an issue, the issue/question is still open, but it doesn't require the Agent to respond. Marking the Conversation as No Reply Needed stops the SLA, which tracks how long it takes for Agents to reply to a Customer.
You don't need to use No Reply Needed in order to close a Conversation with Rules as it's duplicative.