Individualized/Themed Summons list

[sambouchah]

I was looking through Unearthed Arcana and found the Individualized and Themed summon monster lists and was wondering how one chooses what creatures to add to the appropriate list. Is it CR based or maybe HD? If someone could help me figure this out that would be awesome!

Thanks in advance, Sam

[Chronos]

In general, roughly speaking, Summon Monster X will give you monsters with CR X. Be careful, though, of monsters with useful spell-like abilities, as some of those become too powerful in the hands of PCs (you should still have some, of course).