The curtains feature offers three settings that switch the user between a hidden session, password access, or console session.  The hidden session enables a black screen to display so that actions on the remote desktop are not visible.  The "Lock the screen with password protection" locks the screen and displays a black screen on the remote desktop by default, with the option for the user to enter a username and password to view the session.  The console session allows all actions to be seen on the remote desktop during connection, and offers the best graphics performance and resolution options.

The curtains option is only available for iRAPP and therefore is only active if the setting "Allow multiple users to connect simultaneously" is left unselected.  Curtains are not available for iRAPP TS with simultaneous connections. 

The image below shows the iRAPP server Control Panel where the curtains feature can be managed.