These bi-directional meters from Emlite display three readings: Import,export and Net (import deducted from Export). They are needed for PV installations with hybrid battery storage and to replace existing old meters if DC storage is retrofitted.
OFGEM guidance says that any DC coupled battery system on a FiT installation which has the ability to charge from the AC (regardless of whether it is set up or not) must have a bi-directional meter installed in order to claim the FiT.
The ECA2.NV model has a pulsed output.