I think I must to give it last try and to explain the problem in way that even children will understand it. 
Ok. lets say there is a man, and he want to make a child with head, 2 legs and 2 hands. And there is a woman, who want the same as well.
Man is my rig, woman is Epic rig.
Man have a tool with 5 ends and woman have 5 holes. To make a normal child they need to combing all tolls and holes together same time.
But they have a problem. There is a wall between them. Wall is Unreal re-targeting node. 
And this wall have only 3 holes. So when man and woman trying to make a child with had, 2 legs and 2 hands they cant do it.
They need to decide what parts their child will lack.
Hope this explanation will work for every one.