Before April 2014 new homes in England & Wales were only assessed in Part L against a carbon target (DER/TER). This focus only on CO2 emissions tended to skew things in strange ways, such as building with inefficient materials but bringing down the CO2 emissions in the SAP calculation with renewables or low carbon heating systems. As great as renewables are it seemed illogical to have lost sight of the 'energy hierarchy' whereby it is sensible to reduce heat losses before resorting to renewables.
Thus Fabric Energy Efficiency (FEE) was added to the Part L requirement, which in SAP is known as DFEE/TFEE (Dwelling Fabric Energy Efficiency / Target Fabric Energy Efficiency). DFEE & TFEE are the energy demand in kilowatt-hours per m² per year (kWh/yr).
Fabric Energy Efficiency as the name implies is a measure of the efficiency of the building fabric, which in essence boils down to the u-values, air tightness and thermal bridging - the parts of the building that lose heat.