Default user title ladder = based on trophy points.
Use the following user title is what you want for groups such as moderators and define it. Ie, here, the moderators group uses the following user title:
So regardless of what the title should be under the trophy points user title ladder, it will always be XenForo Moderator.
Apply this promotion if...
Content and Achievements > User has posted at least X messages. You can pretty much leave the usergroup at default, just revoke the permission for that particular node, as Jeremy says.