This adapter takes an AN6 male flare on one side and a 3/8 female quick connect on the other, so an AN-plumbed fuel system can plug straight onto a factory 3/8 line. Modern EFI vehicles use push-fit quick connects rather than threaded unions, and when you fit an aftermarket rail, filter or pump, you otherwise have to cut into the standard line and improvise. This adapter avoids that entirely, and being reversible in practice it also makes the return trip easy if the car goes back to standard. AN6 suits the flow demands of most naturally aspirated and moderate-boost EFI engines.