RE: AX 365 EDT relations VS Normal relations

You should not override relations, define it on the EDT if that value is the natural key of your table. It is used if you want to define more complex relations on the table, like on journal lines you typically find AccountCode as an Enum selection field for the type of journal, and AccountNum is the value for the dropdown based on AccountCode.

https://msdn.microsoft.com/en-us/library/aa577032.aspx

http://axwonders.blogspot.hu/2012/08/working-with-ax-2012-edt-relation.html

IgnoreEDTRelation

Used in EDT (Extended Data Type) relation migration.

When migrating relations from an EDT node to a table node, you may skip an invalid relation for a given table field. To do so, set the IgnoreEDTRelation property of the field to Yes. The default value is No.