How to choose a specific item when you have two with the same name and ID

[Magnos]

I have a problem, I created a new environment, with new mobs, but I can't select exactly the item that will drop from the mobs, how do I select a specific item when there are two items with the same name and ID? example a heroic and another mythical

[Shauren]

You cannot select it manually, it will happen automatically, depending on dungeon/raid difficulty (only works inside instances)