Rust free, a term commonly tossed about in advertisements for used automobiles and parts for the same has so many different meanings as to be almost meaningless.
It can mean "no rust that I can see, though I really didn't try very hard, and if you look at the bottoms of the doors or the underside of the car, or heck, any of the usual places, you'll find some significant surface rust". Or it can mean "light surface rust in the usual places". Sometimes, when you speak to the owners of these vehicles, they will repeat the assertation that the vehicle is free of rust, yet when you look at the vehicle, the rust is there, plain as day. I've meant to ask them what they meant, or what they thought the orange colored material on the vehicle was, but I haven't felt up to the fight.
In cars from the 1960s and before, it tends to mean that the item only has surface rust, without any significant pitting or perforation of the sheet metal.
Some cars, of course, are far worse than others. It seems that E-type Jaguars and American cars from the mid-1950s began rusting the instant that they were created, and that the only rust-free examples are those that have been fully restored.
In some situations, used with a bit of humor, it can refer to a car with plenty of rust, but for which the seller is only charging for the non-rusty bits. Along the same lines, it can also refer to a vehicle for which all the rusty sheet metal has been removed, leaving the buyer with a vehicle that needs major work, but for which all the remaining parts lack significant rust.
Sometimes, it can even mean an item that is truly free of rust.
Just beware the term "relatively rust free" - this can mean anything from minor surface rust to something where barely half the original sheet metal remains.
Don't get me started on "new old stock".