A friend who used to work in a hobby shop said it refers to the length of the lead.

I bought a pair of 'surface' Futaba analog servos to drive flaps (high torque, reasonable price)
and the leads are shorter than what I would expect.

The instructions do warn about using 'ni-cd batteries only' perhaps because some people would
use dry cell batteries in their cars which could not provide the current to drive these servos properly
but I checked the current draw & it was nothing too outrageous.

John.