Hund's coupling cases are idealizations.  The appropriate case for a given situation can be found by comparing three strengths: the electrostatic coupling of  to the internuclear axis, the spin-orbit coupling, and the rotational coupling of  and  to the total angular momentum .
For Σ states the orbital and spin angular momenta are zero and the total angular momentum is just the nuclear rotational angular momentum. For other states, Hund proposed five possible idealized modes of coupling.
The last two rows are degenerate because they have the same good quantum numbers.
In practice there are also many molecular states which are intermediate between the above limiting cases.