What do Morgoth and Sauron's [U]s mean?

[Glorfindel]

I have been wondering about this ever since my first winning game. As I am sure nearly everyone reading this knows, Sauron and Morgoth are listed with a '[U]' next to their symbol in the visible monsters list. I have always assumed that this meant that while they were 'p' or 'P' or 'A', for some purposes they counted as major demons. I would rather like to check that assumption now, as it is relevant to my current game.

I am pretty nearly ready to fight Morgoth (I could fight him now if I were not holding out for more items I have never seen before, just out of interest). My current melee weapon is a mace of disruption of Gondolin. I just found a mace of disruption of *Slay Evil*, and while it does not do as much damage to demons as the Gondolin mace, it does substantially more to evil creatures not among Gondolin's specialties.

So, which one would be better for the Morgoth fight? Is he a demon for 'Slay' purposes, or is he just evil?