A CAN bus (controller area network) is the internal data network that lets a vehicle's controllers, such as traction, hydraulics and the display, exchange messages, and it is how add-on systems read or command many forklifts.
Many modern forklifts, electric and engine-powered, run their controllers on a CAN bus. Safety and telematics systems connect to it to read data such as speed, direction, fork height, faults and hours, and on some trucks to send a command such as a reduced speed limit. That is why automatic slowdown is described as working on compatible forklifts: it needs a truck whose controller accepts the command and a manufacturer-supported way to send it.
CAN messages differ between manufacturers and models, so an integration that works on one brand may not work on another, and older trucks may have no usable interface at all. Where there is none, systems fall back to warnings only or to a hard-wired speed input. Writing to a truck’s controller is a modification that can affect safe operation, and OSHA 1910.178 does not allow such changes without the manufacturer’s prior written approval. Ask any supplier how they connect to each truck model in your fleet. The retrofitting guide covers mixed fleets.