" Every base relation should be in its highest normal form (3, 5 or 6th normal form). "
If I remember my database lessons correctly there is no strictly highest normal form. It progresses from 1NF to BCNF, but above that it is more choosing different trade-offs.
Even below it is always a trade-off with performance and that is why we most of the time aim for 3NF, and sometimes BCNF.