A Severe Hit is an attack that currently does a unknown effect on a monster.
Severe DMG means it will Occasionally deal Double or more DMG on a normal hit as well as Weapon Skill. Severe Damage has been tested for, as stated above. More detailed information is located at the FFXI Forum.
Level Name Jobs Bonus 99 Borealis PLD / DRK Occasionally deals Severe weapon skill damage 95 Coruscanti THF / BRD / COR / DNC Occasionally deals Severe damage