Your agents reply to customer tickets by adding comments to them. They can do this either through the Zendesk Support web interface, or by responding to the ticket via email.
You can make these comments either public — in which case they are sent back to the customer — or you can make them private by unchecking the box underneath the comment area. Private comments are great for when you want to discuss the ticket internally with your team, but you don’t want to share that conversation with the customer.
By default, your comments are public — you have to click Internal note if you want your comment to be private. Sometimes you might forget to check it, however, in which case a comment you wanted to keep internal gets shared with the customer.
If this is a situation you don’t want to risk, or if you post a lot of private comments, you can change your comments to be private by default. This means that you have click Public reply above the comment area if you want the comment to be public to your customer.
To do so, click the Admin icon ( ) in the sidebar, then select Settings > Tickets. There, you’ll have the option to deselect the “Agent comments via web are public by default" as well as "Agent comments via email are public by default."
The option to make ticket comments sent in by email public by default is handy if you want to use the web interface for private conversation, but you want to be able to reply by email to the customer.