Send note by default
This is an option, that turns on the checkbox Send note by default. You won't need to check it every time manually. In order to use it, go Administration -> Helpdesk and check the field in.
When you edit the issue, you will notice that Send note field is checked in.
If Send a note is not checked, only Redmine users involved in this ticket will see the notifications about answers (it can be used if you want to have private conversations with team members without notifying the customer). And if this checkbox is checked then the comment text will be sent to the customer - helpdesk contact - as well.