An AN6 inline shut off valve, plumbed into a line so a circuit can be closed off on demand. The practical value shows up during servicing: close the valve and you can change a filter, swap a pump or dismantle a section of plumbing without draining the tank or losing the contents of the system down the workshop floor. AN6 is the everyday size for fuel feed and return lines on most builds, so this is the version most people need. It also has a place as a safety isolator on a race car, or for shutting off a secondary circuit such as a nitrous feed or an auxiliary cooler. Mount it where the handle is genuinely reachable.