I'm not "suggesting" anything. I simply cut and pasted from the available online manuals and used personal experience on the aircraft for the rest.
The gauges show all the measurable fuel in the tanks. There is no regard as to usability or unusability.
As to what customer code the information I cut and pasted comes from it's from generic SDS. The tanks and probes aren't altered for customer customization. The only changes are to software for indication in pounds or kilograms.
When the center tank indicates 0 the tank is virtually dry. I have pumped the fuel out of the tank to the 0 mark and gotten less than 5 gallons out of the sumps at the end.
With the plane flying there is a nose up attitude which keeps the pickups in the fuel. It's almost all usable (barring turbulence).
The tank units are simple capacitors which measure the reactance of their environment. They aren't calibrated to measure anything exotic and the processor doesn't have any setting for a usable or unusable quantity of fuel.
Each Signal Conditioner Circuit Card does, in fact, sum the total of the probe values to measure the total of each tank. Those quantities are summed to give a total fuel aboard value which is sent to the FMCS (if installed) for CG and economy management.
We're disagreeing for a very basic reason. You're looking at the operation manual and I'm dealing with technical specifics. The mechanical indicators of the past didn't have the capability of indicating quantity within a meaningful tolerance to the aircrew. With the advent of digital measurement the accuracy capability is better than 1% so the indications show it all. The crew needs to be aware of what portion of that fuel isn't usable.
The system measures all measurable fuel and that's just about all but the bottom 2" in the lowest portion of the lowest fuel cell compartment.
If the fuel indicator says 100lbs. That's what's in the tank.