What happens to a corporeal undead creature when it is destroyed?

An undead creature is:

Not at risk of death from massive damage, but when reduced to 0 hit points or less, it is immediately destroyed.

What does "destroyed" mean exactly here? Do they just become an inanimate corpse? Do they turn to dust?

My players will soon encounter a curst, a type of undead that, after reaching 0hp, instead of being destroyed it becomes paralyzed until it regenerates later. Since my players could fail the Knowledge (religion) check to identify the creature, I’m wondering if they could realize something is amiss when the curst drops paralyzed (because he isn’t being "destroyed" like normal undead).

I’m specifically looking for something official, not advice on how to house-rule it.