What is the difference:

Plesk Domain User ()   
Plesk IIS User (IUSR_)   
Plesk IIS WP User (IWPD_402())

*account name removed from these

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How do I change file permissions?
Customers have total control of their site for permissions i.e. installing 3rd party scripts if you enable the proper permissions.

You can modify permissions by logging into your account and choosing File Manager. At the right hand side of the screen, an icon resembling a lock is the permission icon. You must set the appropriate permissions such as Read/Write etc whatever the script requires for the IUSR_yourusername account.

We do not provide support for 3rd party scripts other than scripts installed with Plesk.

To add to Gohickey's reply. Plesk IIS User (IUSR_) is an anonymous access account which used by IIS to access pages on the site for incoming requests. 

Plesk Domain User is your user you login as accessing the account in Plesk or by FTP.

Please advise if you have any further questions.