The following standard indicators should be used where data are not entered . “ NE ” ( not estimated ) for existing emissions by sources and removal by sinks of greenhouse gases which have not been estimated . Where “ NE ” is used in an inventory for emissions or removals of CO2 , N2O , CH4 , HFCs , PFCs , or SF6 , the Party should indicate , using the completeness table 9 , why emissions could not be estimated ;