General speaking, when you click "Apply" button to commit the operation on Partition Assistant. The operation will be completed under Windows Mode. As below:
However, some operation could not be completed under the Windows Mode, such as "Shrink system partition", "Clone System Partition" or some applications are running on the partition which you want to resize. The target partition could not be locked, therefore the operation will be completed under the Reboot Mode (including PreOS Mode and Windows PE Mode).
Windows PE Mode
When AOMEI Partition Assistant needs to reboot computer into WindowsPE (WinPE) mode to perform operations. it will create WinPE bootable iso based on system's recovery environment, and then reboot into WinPE environments. After operations are completed, it will reboot into Windows by default.
ProOS mode doesn't need to create ISO, so it will faster into PreOS mode to perform operations. For system partitions, we suggest that you select WinPE mode to resize partitions.
1. Select C: drive to shrink.
2. Click "Apply" to perform operation.
3. When click "Proceed", it will pop up "Reboot required" window with two modes:
1) Check "Enter into Windows PE mode to perform operations" option and click "Yes", it will first create WinPE iso and then reboot into PE environment to perform operations.
2) Uncheck "Enter into Windows PE mode to perform operations" option and click "Yes", it will immediately reboot into PreOS mode to perform operations.
When program reboot into WinPE environment and detect system environment changes, it will quit WinPE environment and then reboot into PreOS mode to continue operations.
If AOMEI Partition Assistant can't detect system's recovery Environment due to some reasons, it will only reboot into PreOS mode.
Manual start Partition Assistant PreOS mode:
In order to make process running under boot mode you should holding down "shift" when you click "Apply" on the toolbar, and then it will commit the operation under PreOS mode.