App pool identity versus impersonation identity?

I found only one thread relating to this but it did not answer the question.

I'm curious to a link or explanation of the difference between setting an impersonation user via in 开发者_开发知识库the web.config versus setting the application pool identity in IIS. They seem to be independent and am confused on the detailed differences. Thanks.

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Using impersonation in the web.config allows you to override whatever identity was configured for the Application Pool the app is running under - it's just a more fine grained method to control identity ( on the app level vs. the ApplicationPool level), so you could have two apps run on the same AppPool, but one of them uses impersonation to use another identity.