Why Table cannot have reference by EDT

In TableA, it has "a" and "b" two EDT, I build index1, and push "a"  into this index1. And Set replacement Key as index1. ("a" is string)

Then I build a new EDT string as "Refa". I set its Reference Table is TableA, Then add a new Table Reference under Refa->Table Reference. Under this Table Reference, it shows "Refa == TableA.a" (Because I set Related Field is "a" (I could not find RecId)).

Then I build TableB. But after I drop "Refa" into the TableB->Fields, the TableB->Relations does not have anything. 

(Both TableA and TableB, I changed Table Group to Group)

When I followed someone's video, after he drops a Ref EDT into a table, it works. However, when I do it by my self, it nothing happen. 

Please, it is a really problem for me now!! Anyone can help me?