The DR (Dynamic Range) determines the lowest ISO possible:

  • The lowest ISO is ISO 200 when the dynamic range is set to DR100
  • The lowest ISO is ISO 400 when the dynamic range is set to DR200
  • The lowest ISO is ISO 800 when the dynamic range is set to DR400
  • The lowest ISO is ISO 200 or ISO 400 when the dynamic range is set to DR-Auto depending on the contrast of the scene.
  • That isn’t just Auto-ISO
  • You can’t set ISO 200 if you have set the dynamic range to DR200 or DR400
  • You can’t set ISO 400 if you have set the dynamic range to DR400

BTW, this applies to all the current Fuji cameras, from the X-PRO1, X-PRO2, X-T1, X-T2, X-T10, X-E…, X100…