Elementary Key Normal Form (EKNF) is a subtle enhancement on 3NF. By definition, EKNF tables are also in third normal form (3NF). This happens when there is more than one unique composite key and they overlap. Such cases can cause redundant information in the overlapping column(s).
A table is in Elementary Key Normal Form (EKNF), if and only if, all its elementary functional dependencies begin at whole keys or end at elementary key attributes.
For every full nontrivial functional dependency of the form X→Y, either X is a key or Y is (part of) an elementary key.
References
Elementary Key Normal Form Wikipedia(Text) CC BY-SA