In the World of Sulerin database I came across a thing called an Aspect, which is described as a being spontaneously or deliberately created by a power.
The encyclopedia says an Aspect is this:
More broadly, an aspect is a particular name and shape taken by a deity before one world or group of worshippers. It is possible for a deity to die on one world, but remain alive on others, because only one aspect of the divinity's many facets has been destroyed.
Those two paragraphs seem contradictory to me. I read the information about Avatars here:
http://www.wizards.com/default.asp?x=d20/article/srd35
and it seems that if an Aspect is an entire face of a deity on a plane, that it is greater than an Avatar. So my questions are:
1. Are there any rules describing more clearly what an Aspect is, especially as compared to an Avatar?
2. Can any power create an Aspect? According to World of Sulerin the only Aspects statted up are for Tiamat, Bahamut, and a bunch of Tanar'ri and Baatezu powers.
3. How many Aspects can a power have at any one time?
Those two paragraphs are contradictory and referring to two seperate uses of the term "aspect."
The second paragraph just refers to "faces" that a power uses to gather worship from different groups of individuals. I'm not really aware of any situations (at least off the top of my head) in which a deity died on one world and remained alive on another. A couple possible examples come to mind, but they all involve avatars. In this use a deities "aspect" is not truly distinguished from a the deity itself.
The second usage is primarily limited to the Miniature's Handbook. This is "aspect" in a meaningful sense of the term (since it distinct from the deity itself). They were largely created to justify the creation of minatures of deities and archfiends, and allow adventurers to defeat (in an extremely limited sense) these beings.
For answers:
1. There are examples of aspects but no clear rules on creating them. Whereas avatars are direct extensions of a deity, aspects are more independent. Aspects generally have a CR somewhere between 10 and 20, with some exceptions being as high as 27 (in the case of Asmodeus). Avatars, going by the official and broken WotC rules (they cause DvR inflation among other things), generally have (if calculated) a CR in the 30s. I could try to answer more specific questions on this.
2. To the best of my knowledge, any power can. Archfiends, celestial paragons, and other cosmic entities can manifest aspects but not avatars.
3. There are no rules that state this. But they can definitely manifest more of them than they can avatars; so probably "alot" (dozens perhaps). To distinguish them more clearly from avatars, I think aspects might be very temporary manifestations of a power or cosmic entity created for a specific limited goal or at the calling of powerful clerics.