Mike, your second post's research pretty well summed up what code is. In model railroading code is the height of the rail. If I'm not mistaken code 100 is 0.100". All other codes follow that formula.
In real railroading code refers to pounds per yard, so a 3' section of code 100 weighs 100 pounds. Back in the days of sectional rail the standard length was 33', so a full length piece of code 100 weighed 1100 pounds.
Both tinplate O and S enthusiasts don't usually talk about code. They both need tall rail for the deep flanges on their wheels, perhaps on the order of .250" or code 250, which is unrealistically tall.
If you plan to mix codes when modeling, you can make your own adapting joiners. If you want to go that route, usually what you do is take a joiner of the larger rail and crush one end, shimming if necessary, then soldering the lighter rail in place. I see from the previous post that Atlas makes 100 to 83 joiners, but that may be the only size available.