This read-write integer identifies the filter group applied to downstream traffic destined for subscribers behind a given cable modem, defaulting to zero (use default classification) unless set by the modem's provisioning object during registration. Operators use it to steer or restrict traffic aimed at a subscriber's devices according to a specific filter policy. For example, setting this to a nonzero filter group index would apply that group's classification rules to all downstream traffic headed toward devices behind that modem.