uh.. undead?

if skeletons and undead are classified differently.. if they DONT have flesh and their name doesnt have skeleton, they are undead? or does this undead mod only regard bone minions and such?

and the creatures in UW, are most of them "undead"?

Yes
I cant answer the mod thing,
and No.

Undead ranges from walking skeletons to ghools. For example, a skeleton dosen't have flesh but a zombie does. Both are Undead because both died before being reanimated. Undead also consists of ghosts like phantoms and such.

The creatures in GW range from giant insects to other people, undead people, mythical beasts and other mythical beings like dwarfs.

soo.. EVERY undead creature is affected by the undead dmg mod except for skeletons? because there is a different mod for skeleton damage.

Most things that have something suggesting they're undead in their name, without having flesh and not being able to have their corpses manipulated are counted as undead.

UW/Fissure mobs would be considered "spirits" of a sort and would/should not fall under the "undead" label.

Think of them as demon's or summoned spirits of evil, versus undead which are "raised" and controlled.