Under Rail or Over Rail

What is the difference? Sorry for the dumb question

It's a difference in the design of the tail section of the chassis. Under rail has frame rails (or at least tubing) that literally runs "under" the rear end tubes. Over rail does not have those, only frame rails "over top" of the rear end. An under rail car would arguably be stronger in the rear.

Thank you for clearing this up for me

Is one better than the other handling wise? I have heard under rail is better because it does not allow/have as much chassis flex and allows the springs/shocks work better. I have also heard over rail works better because it allows chassis flex lol so I have always wondered.