This is the amount of fuel, in this context meaning the combined fuel, oxidizer and possibly propellant, remaining right after the spacecraft reaches orbital velocity at its altitude. If this amount of fuel is added to the payload ratio and another analysis is performed; the spacecraft will barely reach orbit with no fuel, or miss it by a tiny amount because of rounding error. Sometimes this altitude is below the low drag altitude and in any case maneuvering fuel is needed, so it is good to have an extra one twentieth fuel compared to the empty mass of the spacecraft.
 

This is used in the trajectory tables in atmospheric spacecraft, multi stage spacecraft, and spacecraft cost.
 
 

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