See also: In-Area Import Fraction. Regional Import Fraction
(%) Fraction of total in-area trade in a fuel met by exports from the selected region.
This variable is only visible in multi-region areas. Its values should be between 0% and 100%, and should sum to 100% across all regions for each fuel. In addition, it is only visible when you enable Allow Trade Among Regions: an option on the Calculation tab of the Settings screen. You will also need to edit the Fuels screen and specify which resources can be trade among regions.
This variable works in conjunction with the In-Area Import Fraction variable which creates a demand for internal trade within a multi-region Area. Once this internal trade has been calculated the In-Area Export Fraction is used to specify which of the regions meet that demand by exporting fuels to other regions in the area.