This policy setting specifies whether Windows Installer RDS Compatibility runs on a per user basis for fully installed applications. ...

This policy setting specifies whether Windows Installer RDS Compatibility runs on a per user basis for fully installed applications. Windows Installer allows one instance of the msiexec process to run at a time. By default, Windows Installer RDS Compatibility is turned on.
 
If you enable this policy setting, Windows Installer RDS Compatibility is turned off, and only one instance of the msiexec process can run at a time.
 
If you disable or do not configure this policy setting, Windows Installer RDS Compatibility is turned on, and multiple per user application installation requests are queued and handled by the msiexec process in the order in which they are received.