the AFR is the source of fuel for cruising around the local system in SC and once depleted it draws another full load from the main tank
the main tank provides fuel for jumps between systems and to top up the AFR
you can arrive in a system, after multiple jumps, with a bone dry main tank but have the fuel to fly to a station in that system to refuel ( a reserve) ... I would not recommend this
note ..........the AFR is NOT re-filled during fuel scooping at a Star
the AFR can only be filled at a Starport or outpost which provides refuelling service
all in all it does have a role in the gameplay of the game as a whole and something that you need to monitor on your travels
hope that helps
