It is only from overworld mobs. Other than that, anything goes, but as I have said before, there is no benefit to farming like “the most mobs per hour” or something. Every 60 seconds if you are doing overworld content there is a hidden roll. If you hit on it then the next overworld mob you kill will have the drop. So as long as you are killing something at least once per minute in the overworld, you are getting the chance to get it.