JB stands for the junction box where electric wire junctions are gathered into one group and housed. JBs contain PCB-like bus bars (metallic connection chips) in a block, provide the capability to connect circuits in a concentrated manner, and have related parts built in, such as relays, fuses, and circuit breakers. An A-JB (active junction box), another type of junction box, contains parts that include electronic circuits, such as ICs.

ECUs (Electronic Control Units) for vehicle bodies use information that enters junction boxes to control door locks, the lighting system, and LAN communication.
Today, the LAN capability that contributes to reductions in the number of wiring harnesses has become increasingly important with the current focus on vehicle weight reductions, and vehicle-body ECUs are now developing from a mere component into a significant LAN center in vehicle systems. Awareness of this new concept of the LAN system has become imperative in the ECU design process. Our challenge is to further the evolution of vehicle-body ECUs into a total system that includes wiring harnesses and junction boxes.