Users are unable to launch multiple instances of published application when TWIMode is set to "Off".
 

Users can still leverage the Shift + F2 hotkey and switch between seamless mode and windowed mode as needed. If multiple instance of the application needs to be launch in windowed mode, first launch in seamless mode and switch to windowed mode manually.

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Problem Cause

From XenApp/XenDesktop 7.x, the behavior of launching Windowed mode sessions inherits the behavior of RDP "Start the following program on connection" feature.
 
Users cannot launch multiple instances of Windowed mode session if the session is launched in fixed window mode.
 
This was not the case for Pre 7.x. Users were able to launch multiple instance of Windowed mode session in XenApp 6.x.
 
The following difference can be observed in CDF trace between 7.x and 6.x when a subsequent session is attempted to launch.
 
XenApp 6.x：
Can the session be shared? -> Cannot share this session as the existing session is Windowed mode -> Launch the subsequent session in NEW session.
 
XenApp 7.x:
Is there a session from Same user using the same application -> Yes, will not launch a new session as session already exists. -> subsequent session is discarded.