What in your opinion affects success?

I'm particularly interested in summoning from different areas. I got asked to summon some elite dwarven guards away from the fort recently and I failed utterly.

Does anyone know what the summon command latches onto?

To explain, when I type co summun "elite", does it go for some numbered elite guard, or the nearest thing which has "elite" in its label.

And are there some non-aggro, non-cabal mobs that you just can't summon at all?