HAL's definition of formal for gentlemen, at a minimum, is tie and jacket. (obviously they expect shirt and trousers...... one smart guy on another pointed that out when some of us referred to only tie and jacket).
The ships are inconsistent about enforcement.
We have seen men seated without the tie and we have seen men turned away without the tie.
A blazer, shirt, tie, trousers and shoes guarantees you will be seated in MDR. Easy.
For many years and a great many cruises, DH always brought his tuxedo. No more. He now wears a dark suit..... and on a rare occassion has even worn just a blazer/tie.