The quote below from the FCTM (AO 24) gives you the philosophy:
"The electrical distribution has been designed to fly, navigate, communicate and ensure passengers comfort. The ELEC EMER CONFIG SYS REMAINING list is available in QRH. The significant remaining systems are:
Significant remaining systems in ELEC EMER CONFIG
FLY; PFD1, alternate law
NAVIGATE; ND1, FMGC1, RMP1, VOR1/ILS1, DME1
COMMUNICATE; VHF1, HF1, ATC1
On BAT, some additional loads are lost such as FAC1 and FMGC1"
So they have allocated the electrical demands above to the busbars which remain powered in the Emergency Electrical Config.
mcdhu