From Airbus 'Getting to Grips with Airplane Performance':
To minimize overall fuel consumption during flight, a low cost index must be used. As the descent phase is performed at idle thrust, it is advantageous to maximize its duration, from a fuel consumption standpoint. This is achieved at a low descent speed, which depends on the aircraft type (e.g. 250 knots for the A320 family). In any case, the descent speed must remain above green dot.
CI = 0 ⇒ IASECON = Minimum descent speed (depends on A/C type)