"Pause Windows when no applications are open" feature is designed to save your Mac resources. Pausing a virtual machine releases the resources, such as RAM and CPU, currently used by this virtual machine. Released resources can then be used by the host computer and its applications, or by other virtual machines running on the host computer. If you select this option, the virtual machine will be automatically paused*, provided that:
the virtual machine is running, but its window is not active for more than 30 seconds.
- there are no applications running in guest operating system (no icons of the guest OS applications are present in the Dock).
To manage this feature:
In Parallels Desktop 11: Virtual Machine Configuration > Options > Applications
In Parallels Desktop 12: Virtual Machine Configuration > Options > Optimization
* - This option is available only when Parallels Tools are installed in the virtual machine, and Isolate Mac from Windows option is disabled in the Security pane.