Using the leave ticket open by default option in My Account

Set the default status after replying on a ticket
Please note each user will need to set this option within their own account settings 

01 Reply box

When the option is selected, the Close ticket checkbox inside the Reply box will be unticked and the ticket will be marked as Waiting when a message is sent 

When the option is not selected, the Close ticket checkbox inside the Reply box will be ticked and the ticket will be marked as Resolved when a message is sent 



02 Using the option

This option allows agents to decide when a ticket should be marked as Resolved
Once the agent is happy that the issue has been resolved, they can mark the ticket as Resolved

To avoid tickets building up in Waiting, you can set a number of days after which tickets will be marked a Resolved
For more information on this settings, see here