It is the total length of delivery tubing, from the tank to the NV.

Remote carbs have an additional problem. The main pressure drop component is the needle valve, which in a conventional carb is directly upstream of the throttle bore, where the lowest system pressure is found -- the pressure differential between this locus and the tank, provides most of the driving head for fuel delivery, which is helped by active tank pressurization. The low throttle bore pressure also assists in fuel vapourization -- a good thing for well controlled combustion.

With a remote NV, the delivery tube to the carb inserts an additional frictional loss downstream of the NV and ahead of the throttle bore, sometimes resulting in a line pressure that falls below the vapour pressure of the fuel, resulting in flashing in the delivery line (bubble formation), which interupts fuel delivery & results in erratic tuning.