Activate the Automatic option if you want the Maya FBX Plug-in to automatically identify, set, and convert units of the incoming 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 and scale factor. If you disable the Automatic option, the Scene units converted to option becomes available.
When Automatic is active but the file units match the system units, there is no need for scale conversion and the plug-in applies no conversion. The displayed Scale Factor is 1.0 in this case.