The first three models tell you how many interfaces that you get. Single has one interface, double has two, etc. Their are different insterfaces for different manufacturers. If you have a Ford and a Toyota, you would need two interfaces. OBD-II is standard on pretty much all cars 1996 and newer.

Here are some notes from the www.obd-2.com website:
Quote:
Your choice of one of three possible OBDII gateway router interfaces:
ISO - for European, Asian, Chrysler
VPW - for General Motors
PWM - for Ford
Notes:
98+ Concorde, Intrepid, LHS, 300M, 2000+ Neon, Isuzu, 96-97 Toyota, Celica, Supra Turbo use VPW
96,97 Probe 2.5L, 96 Tracer 1.8L, 96 Escort 1.8L, Triumph, Geo, Catera, 97 Paseo, Camry, Avalon, 1999+ Celica and Supra use ISO
For instance, I want to be able to hook up to any OBD-II car, so I want to get the Tri-Com. It has every possible interface.

I hope that this helps.

96 Contour GL (2.0L Zetec, ATX)

Sony CDX-C780 HU, Sony CDX-805 10 disc Changer, Sony XDP-210EQ, RF Punch 100.2 Amp, RF Punch 240.4 Amp, Dual 12'' Punch subs, Four Infinity Kappa 693.3i 6x9's, Optima Yellow Top

CTA Intake, Hood Struts, EL Ford Logo, Hi Beam/Low Beam Mod, Blue LEDs in door handles and footwells