The others have answered one way of interpreting your question, and I'll answer interpreting it another way.
Depending on the axle loading, the real railroads use heavier rail as the tonnages go up. The heaviest ever used in N. America was used by the Pennsylvania Railroad, and it tipped out at 155 pounds/yard in linear measure. Code 100 rails are closer to what would be 165 pound rail, which nobody ever used, except perhaps for overhead cranes in heavy foundry and machine shops, say where locomotives are being erected.
Much closer to real rail is Code 83 which is about 130 pound rail. That was, and often still is, main line weight on current Class One railroads in N. America. But it's not by any means standard or universally used from road to road.
Where axle loadings are under about 17 tons, say on industrial spurs or on short lines using lower tonnages, it might be Code 70 rail, now down to about 110 pounds/yard (I'm guessing...haven't really calculated it, but you get the idea).
In yards where rails are embedded in hardpan, or where axle loadings aren't so high AND speeds are low, and for streetcars, and industrial sidings where lighter goods are moved, say bales of cotton, the weights were lighter, maybe down to as low as Code 50 for the models.
This is just an explanation for WHY WE HAVE CODE RAIL at all, and not to be taken as definitive.
When you are choosing a code, make sure that it will allow for clearance of your rolling stocks' wheel flanges over the small spikehead details. As your code gets lower, the wheels ride lower and closer to the gravel, grit, ballast, mud, and spikes.
Mesenteria did a great job of explaining the
track codes for HO.
Probably most of us use the more readily available
code 100 track. Almost any Hobby Shop will have
a large stock of code 100 flex track and the turnouts or
crossings that are compatible to it.
Those who prefer a much more
realistic look choose from those other codes that more
closely represent real railroad tracks. There are also
compatible turnouts and crossings for these codes.
tho you may have to shop around to get what you want.