[Arragonis]

DFCO in my part of the world is Deceleration Fuel Cut Off - basically the FI system determines that the driver has the throttle closed and the engine is being turned over by the car's travel and it shuts off the fuel injection and ignition.

There may be a lower RPM limit - e.g. my engine won't go into it under 1400 RPM (from my experiments - it may be different) but once in that mode I can go slower and the engine won't add fuel until the engine speed us under the idle speed ~ 870 rpm.

Carbs don't - they just spray in whatever based on the air flow - SU carbs are the most efficient for FE as far as I know due to the rising piston design - not perfect by any means though.

EDIT - to answer your question better - assume you are descending a hill and in top gear you can coast even with engine braking. The engine is being turned by the car so the Fi system shuts off fuel and you get infinity MPG and your car's systems (PAS, ABS, EBD etc.) are still powered.