How is a mob flagged as "Undead"

[TigerFeet]

I'm currently working on the PEQ database rebalancing spells for a more "classic era" feel. Some spells only work on: eg undead (target type 10 if I remember correctly).
How does the server code determine if an NPC is undead?
There isn't a "target_type" field in the npc_type table entry.

Is is done through the "race" field. Looking at the possible races there are many entries for skeltons, ghouls, mummies etc. Does the code check against all possible undead races rather than check a single table entry?

I feel I'm missing something here?

Can someone please point me in the right direction?

Thanks, Andrew