Activate the Automatic option if you want the Maya FBX plug-in to automatically identify and set the units of the destination file to match units of the host application

When you activate Automatic, the Scene units converted to option is disabled, although it shows the resolved unit & Scale Factor. If you disable the Automatic option, the Scene units converted to option becomes available.

When Automatic is active, the File Units will match the system units, and the plug-in applies no conversion. The displayed Scale Factor is 1.0 in this case.