These systems incorporate the preferred optic technology (red and black 2 wire) using top overfil as well as bottom retain sensors. Retain sensors inform the terminal if the compartments are empty or if product has been retained. The system offers true load at any rack compatibility.R/OM Type Retain / Overfill SystemRed and Black 2-Wire Retain/Overfill System (top only). FloTech brand overfill system kits provide everything needed to install an overfill system on your tank. Top Only Systems include: 1 FT204P monitor, 1 FT202 sensor per compartment, 1 FT401 100' cable and 1 FT402 (20 pcs.) strain reliefs. Retain Systems include Retain Sensors, additional strain reliefs and wire needed. ***Sockets sold separately.*** 7" and 12" probes shown. Special length probes are available.