This sets the lambda (wavelength) number of the preferred first ring, relevant on a DWDM optical ring system where a single physical port can carry multiple distinct wavelengths, each effectively a separate logical ring path. This lets an admin pin a circuit not just to a specific slot and port but to one specific wavelength channel among potentially many riding the same fiber, which matters because different wavelengths on a DWDM ring can have different latency, capacity, or protection characteristics. For example, on a port carrying 8 DWDM wavelengths, setting this to lambda 3 routes the circuit specifically over that one wavelength's ring path rather than any of the other seven.