Hi Michael,
This parameter is there to add some length to the pipe only in the equations that calculate the friction loss, see documentation here. It uses the same friction factor computed from the Haaland approximation or the specified value for laminar regime or a blend of these two values. This "length" doesn't add any volume to the pipe. It's designed to model minor additional losses like some bend in the pipe, connection losses, etc., without using an extra block. It's best tuned if you have pressure loss and flow data over the pipe.
