Here's a simple question I've been stuck on for a while.

When I set < identity impersonate=true > in my web.config so that asp.net impersonates the logged on user automatically (or the anonymous account if not using Windows Authentication), where does the identity that asp.net impersonates come from?

This document: http://msdn.microsoft.com/en-us/library/ff649264.aspx shows three places you can retrieve information about the logged on user:

It seems that none of these locations consistently match the identity that gets impersonated when I set < identity impersonate=true > in web.config.

I would like to know where the impersonated identity comes from.