The FEI is defined as the ratio between the electrical input power of a (fictitious) reference fan FEPref and the electrical input power of the real fan under consideration of FEPact i.e.
FEI = FEPref / FEPact
Both input powers can be accurately calculated according to AMCA 208-18 under §5.2 and §5.3 at any given operating point (flow rate and pressure).
Therefore, the FEI is a relative measure for assessing the energy efficiency of any fan at the same operating point against a fictitious reference fan.