ASP.NET identities (IIS_WPG, IUSR, etc.)

Hello everybody,

I can't find a better place to discuss about a quite the issue I still can't fully understand despite reading many material found on the web.

So I'd like to understand exactly under which identities an ASP.NET application is running, especially whenit comes to access files and folders (which need ACL fine tuning). One could expect to be NETWORK SERVICE (IIS_WPG group) or IUSR_machinename, etc. but what happens exactly, and under which condition ? 

Let fisrt assume the environment is IIS v6 and ASP.NET 1.1

The question is what are all possible scenarios and for each of them, which identity is used ?

Thank you not to post too obvious (and not so relevant) links (I've already done quite a lot of "Googling"). ...