Courtesy of Justin Fore at Fore Innovations LLC:
Gasoline EFI vehicles can be either return, electronic returnless, or mechanical returnless type fuel systems. Return type: A supply line and a return line exists. Fuel is fed to the motor by the return line, and unused fuel is returned back to the fuel tank with the return line. Electronic Returnless type: Only a supply line exists. The fuel pumps are modulated by the vehicle's computer to supply fuel to the motor based on fuel demands Mechanical Returnless type: Only a supply line exists. The fuel pumps generally run full time, but the system pressure is maintained by a mechanical regulator located in or near the fuel tank.