In the Basic Settings section of a website in IIS 7.5, there is an option to specify a user account via `Connect As...`. If this is kept off, pass through authentication is used.

I assumed that would use the identity of the Application Pool however this was not the case. In order to get my website to connect to MSSQL under the correct user account, I had to specify the user under the `Connect As...` screen.

So what is the difference between `Connect As...` and the Application Pool Identity?