Get used to it! There are literally 1000s of things that can be queried by the engine that isn't available to scripters. Some can be found out via workarounds , though some workarounds are often little more than guesses...
Why not just put the dog on a seat by moving him into the vehicle just as you would a man? If you attachTo a dog into a seat position, then that wouldn't stop a man from sitting in the seat at the same time
It is often best to explain what you need something for ("How do I put dog in a car seat?"), rather than asking for an abstract technique ("how do I find out seat positions?") which may not actually be the right way. That way, we can guide you on a better path.
A general comment about attachTo (not directed at original poster): Although this command is absolute gold to everyone who struggled without it in A1, it isn't the answer to all problems...