Geof says:

Mario,

The code number refers to the track height with Code 100 being ‘higher’ than Code 83. You can mix the two but you’ll have to fabricate a rail joiner that will mate the two types smoothly. In HO gauge, you can flatten half of a regular rail joiner and solder it to the under side of the Code 83 rail. The unflattened end is then slid onto the Code 100 rail in the normal manner thus joining the two sections. Note that some tinkering may be needed to get a smooth joint.

As you may (or may not know), the advantage of using the Code 83 rail is that it has a lower profile than the Code 100 which gives a more prototypical appearance to your track. The down side is that SOME of the cheaper and/or non NMRA approved equipment (both rolling stock and locos) have over sized wheel flanges which may make contact with the rail spikes on the track resulting in derailments or generally poor performance. To correct this problem, one would have to install NMRA approved trucks or wheel sets which, though fairly easy to do on rolling stock, could present a real problem with powered equipment. Not impossible, mind you, but it can get difficult.

Hope this helps,

Geof