The Maximum Duration slider sets the maximum number of minutes that can elapse between Auto Saves. If you do not save the project yourself within this time, ZBrushCore will then do so automatically.
Any save, either manual or automatic, will reset the time. This keeps you from encountering a situation where you save your work only to have to wait shortly after that while ZBrushCore performs an Auto Save.
The Rest Duration slider sets the maximum number of minutes that ZBrushCore can be idle before it will execute an Auto Save. If you set this slider to 1, then ZBrushCore will watch for one minute of inactivity. If you then leave your computer, or switch to another application, for that length of time, ZBrushCore will execute an Auto Save.
The Rest Duration is totally independent of the Maximum Duration setting. If you save your work immediately before stepping away from your computer, ZBrushCore will still do an Auto Save after the specified Rest Duration.
Delete QuickSave files
When the Delete QuickSave files button is pressed all QuickSave and AutoSave files will be deleted from your computer. Note that the files are deleted completely, not sent to the Recycle Bin/Trash.