3NF still allows some redundancy when a dependency points into a key from something that is not a super key. BCNF closes that gap by requiring every determinant to be a super key. Aim for it when such a dependency exists, but know the price: the split may lose dependency preservation, which is why 3NF is sometimes kept in practice. "Difference between 3NF and BCNF?" is a frequent interview question, best answered with one small example.