Just for clarification, you have to read 25.1337(b)(1) and 25.959 to understand the intent correctly.
Sec. 25.959
"Unusable fuel supply.
"The unusable fuel quantity for each fuel tank and its fuel system components must be established at not less than the quantity at which the first evidence of engine malfunction occurs under the most adverse fuel feed condition for all intended operations and flight maneuvers involving fuel feeding from that tank. [Fuel system component failure need not be considered.]"
The unusable quantity of fuel in the tank is determined at the most critical attitude during which the tank is expected to be used (typically cruise for auxiliary tanks and low fuel go around attitude for main tanks), then the FQIS is set to read zero in level flight at the fuel level corresponding to that unusable fuel volume in level flight. This will typically mean that, if you are in level flight with no longitudinal acceleration, there is some additional fuel in the main tanks below the zero indicated level that is accessible by the pumps. Operators are not permitted to take credit for fuel below the zero indicated level toward their required mission or reserve fuel.