When you edit the default Approval/Decline email, the new version becomes the default used for all future approvals or declines (until you edit it again). The Approval/Decline emails are across all titles and all members.
To edit the Default messages, click "Approval Email" or "Decline Email" from your Dashboard.
If you wish to create a custom email for a particular title, see here.
Take a look at some sample messages:
Templates for APPROVAL emails
Templates for DECLINE emails