The SDR-FS is the original e-collar receiver for the FieldTrainer SD-400 and SD-400S. For replacement or to add additional dogs to your existing SportDOG Brand system. For maximum range offered by your model, choose the original e-collar receiver for your transmitter unit, but all SportDOG Brand FieldTrainer, SportHunter, WetlandHunter, and ProHunter transmitters and receivers are compatible/interchangeable (actual ranges may vary when mixing transmitter/receiver models).