This ability makes the creature impervious to most types of damage. Any damage dealt to the creature that falls below its massive damage threshold doesn’t reduce its hit points, unless that damage is of a type it is specifically vulnerable to, as mentioned in the creature’s description. Massive damage that doesn’t match the creature’s vulnerability reduces its hit points, but such damage automatically heals at a fixed rate, as detailed in the creature’s description. When the creature takes massive damage from an attack type it isn’t vulnerable to, a failed save renders it dazed for 1 round (instead of reducing it to –1 hit points).
Regenerating creatures can regrow and reattach severed body parts. Severed parts that aren’t reattached wither and die normally. Regeneration continues to work no matter how low the creature’s hit points drop, restoring lost hit points from any damage other than from attack forms the creature is specially vulnerable to.