We know that Microsoft IIS 7.0 offer a slew of authentication methods such as Anonymous Authentication, Form Based Authentication, Digest Authentication, etc.

I read from Professional IIS 7 published by Wrox that:

When we use Anonymous Authentication, the end-user does not supply credentials, effectively mak- ing an anonymous request. IIS 7.0 impersonates a fixed user account when attempting to process the request (for example, to read the file off the hard disk).

So, what is the fixed user account impersonated by IIS? Where can I see it? If I don't know what this account is, how could I assign proper permissions for the clients who are authenticated as anonymous users?

Thanks.