IIS File permissions

Many times you need to give permissions to modify files for web-sites running on IIS.
If you are using the Application Pools identity, you’ll have to use the following usernames:
Set the domain as the local machine. For user type IIS AppPool\DefaultAppPool or IIS AppPool\<AppPoolName>.

Source: http://www.iis.net/learn/manage/configuring-security/application-pool-identities